The two that synergize best are /Fire because Fiery Embrace boosts all your damage from your Primary as well and /Psi because Drain Psyche helps against tough targets. They're also #1 and #2 with respect to AoE damage.
About the post above me, Thorns is definately up there if not before psi. It has 3 actual AoEs, (fling thorns, thorn burst, and ripper) plus thorntrops, which is AoE slow plus dmg over time. I've heard good things about fire/thorny
Fire/Thorns is my favorite toon to date for the reasons above. It has great synergy.
personally I can vouch for fire/energy being fun,
-Fire pretty much solves whatever AE problems energy has,
use flashfire + ae immobilize, let your imps handle the small fries, while you take on the boss or Lt
-the graphics mesh well thematically IMO
It may not be the strongest combo out there, depending on what you want to do, but I'd say its a fun and effective build,
my fire/energy dom besides being my first villain 50, has always been one of my favorite toons, and more so now
I'd say Fire/Fire because of what MinMin said. A large chunk of my damage comes from the DoTs from the primary (Fire Cages, Char, Hot Feet), so Fiery Embrace does come a big difference.
Either way, I can't imagine anything not working with Fire, especially if you end up getting perma-dom. Everything will be Held indefinitely, you won't have any trouble ripping through any mob.
I wont really speak to Synergy, just fun.
Fire/Energy (40) is a whole ton of fun. It is my current infatuation. It is a lot better than it used to be. I had been parked at level 35 forever because it was a bit of a drag to get from 1-35 before the changes, tho I somehow managed. Now, the set hits somewhat hard and you can actually form a good attack chain with Power Push. Yummy! Oh, the set also looks cool with the Dark Cherry Red combined with the fire effects.
Fire/Thorns (50) is my overall favorite so far. Some of it was just the character concept, but still, it is unmatched as yet when it comes to actual FUN. The ripping sounds are great, and I felt like the powers did pretty good damage from early levels even before the buff. Sometimes with some of the more elaborate animations, it can feel a bit slow, but it is still fun to play.
I am also finding Fire/Electric (35) to be pretty darned fun. Static Discharge is simply awesome now that I have it slotted up some. Thunder Strike already hits like a truck, even though I currently only have an ACC in it. Unlike most Dom players, I prefer ranged over melee, so I have only been using Charged Bolts and Lightning Bolt, but they were plenty enough for me. I will start taking/slotting the melee attacks now (starting with Thunder Strike) and I'm sure that will add a fun new dimension to it. The elec graphics really fit the look of the fire, too.
Fire/Fire (40) is also fun, and I like it a lot because you can easily skip Incinerate, and I do like to skip it, even though the damage is good, because its the only melee single target power in the set, so basically Fire/Fire makes a great Ranged Dominator if you are so inclined. Fiery Embrace really makes this combo rock, on both the primary and secondary. Flares feels a bit "meh" now but what are ya gonna do? 
Still a fun set.
I don't quite like Fire/Ice (41)as much as I thought I would. It is endurance heavy, and for some reason the damage just doesnt feel very ... oomph. Ya know? Also, I find the timing of power availability to be a bit of a drag. Remember, I prefer ranged powers early on and melee ones later, and Ice stacks a lot of its melee powers early in the set, and since Ice Bolt is "meh" you cant ever really have 3 good ranged attacks from the secondary alone. (I guess Electric cant either but it doesnt FEEL as rough to deal with as Ice, probably because you get the ranged stuff early in Elec.) Anyway, Fire/Ice is OK. Adequate. And to be fair, Power Boost is nice for the Flashfires and Cinders, though sometimes I do feel its almost a necessity for Ice, which either kills a bit slow, or else I suck at using effectively. I DO like how cool I look in a trenchcoat and duster, while rocking the Hover + Ice Sword Circle. That animation is the saving grace of the set for me.
Fire/Psy (50) used to be better for AoE and its shifted to less of an AoE set, so my emotions are all mixed up about that. I'll need time to re-evaluate. It used to be fun. I know the Psy Shockwave was over-powered, but by being over-powered, it did enough damage and had enough range that it could function as a single target filler attack in conjunction with Mind Probe and Subdue. Now that it hits real soft and has almost no range for an AoE compared to what it used to, I'm going to be forced to drop Ball Lightning and shift my powers to pick up Mental Blast, which I hated before. I'm not sure if Mental Blast was buffed. Subdue still feels weak to me, and Mind Probe is ok but not uber. I probably dont like the short range, tho it looks cool at least. So, I'm not confident that I will find Mind Probe + Mental Blast + Subdue to be satisfying. I will just have to respec and try it after I save up some cash on my character (new build will be tighter so gotta purchase some expensive min-maxy frankenslotting to fit it all in). So ... blah. Not sure here.
Lewis
Random AT Generation!
"I remember... the Alamo." -- Pee-wee Herman
"Oh don't worry. I always leave things to the last moment." -- The Doctor
"Telescopes are time machines." -- Carl Sagan
Nice summary Unicycle, was interesting to read. Thanks for writing it.
One of my projects for i15 was to level a Fire/Thorn domi to replace my old Fire/Psi. Spines is one of my favorite scrapper sets, so I think I will be fine with Thorns animations.
Anyway I never started this toon, but your description might make me reconsider
I love Fire/Thorns. It's the closest thing you can get to the oft mentioned melee mastermind. Once you've got Hot Feet running with the Imps you can Flashfire a mob and jump in with your little minions and just start slashing away as they burn. It's stupid great fun.
Also, I have to say it's probably about the most evil feeling toon I've got (possible exception of my Traps MM due to Detonator). Enemies are left coughing and choking at your hold, and while they're struggling to breath you run up and stab them with a giant spike. Most of the time you can be even more wicked and not even bother to finish them off. Just stand their and wait for the poison to finish the job.
I'm gonna throw another hat in the ring for Fire/Thorn. Add Ghost Widow in there for Soul Drain and Dark Obliteration for more AoE goodness. Everything dies, painfully and quickly.
Flashfire > Fling Thorns on the way in > Soul Drain > Thorn Burst > Dark Oblit. > clean up what's left with a sponge. I don't even bother with Fire Cages, but I'll probably throw it in once I get it slotted and proc'd out.
If you're so inclined you can proc /thorny and really amp it up. All of my attacks have at least one proc in them, and in the AoEs more.
Now if they'd just give us quills in place of Thorntrops...
@ Sfort
Post may sarcasm
Leet speak, and generally difficult to read posts will be not be read.
OAS! AAS! LLS!
Thanks for the great replies everyone, especially Lewis for that in depth breakdown! I'm amazed you've played all of the combos to such a high level.
As it turns out, while stuck in indecision about this dom, I made an Earth/Fire dom . He's only level 10, but quickly becoming quite fun. As for a Fire/ dom, I still want to make one! I was leaning towards Fire/Energy, but now am quite intrigued by Fire/Thorns.
Quick question before I get back to work: Do the thorns redraw if you're alternating between fire and thorns attacks?
Edit: I'm also wondering what people think of thorntrops... worth getting? worth slotting? howso?
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Quick question before I get back to work: Do the thorns redraw if you're alternating between fire and thorns attacks?
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Yes, but it's a relatively quick redraw animation in my experience. The nice thing about Fire is that you're not going to switch back and forth all that often. One or two Fire mezzes to start things off and from there the only time you redraw is if you need to reChar something.
I also have Earth/Fire and he's really fun; totally different experience really. It plays a lot slower and safer in my experience and you tend to hang back at range rather than charge into the fray.
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Thanks for the great replies everyone, especially Lewis for that in depth breakdown! I'm amazed you've played all of the combos to such a high level.
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I didn't think my breakdown was that great. It was just whatever rambling thoughts I had based on my experiences. As usual, I tend to go into more detail for sets that I found issues with. It is just my nature.
As for having played so many, it started hero side. I have also played up EVERY fire controller combination to 50. All of them were played up for real with no power leveling at all, with the exception that my Fire/Thermal got to 32 pretty quickly in a few crazy teams over a period of about 1-2 weeks, which is faster than I'm accustomed to.
Anyway, Fire Control has always been an obsession of mine, and Controllers generally. Despite my huge barn filled with Fire Control characters, Earth Control is my favorite.
Anyway, so I have LONG since had all my Fire Doms created and slowly being played up here and there, with no particular rush. I play them as infatuation allows, but I play very frequently so eventually they all get up there.
I also have a Plant/Fire Dom in the 40s and an Earth/Fire in the 40s. I also have tons of other Doms in the 11 - 35 range or so.
Anyway, I'm just rambling. Tired, I guess.
Lewis
Random AT Generation!
"I remember... the Alamo." -- Pee-wee Herman
"Oh don't worry. I always leave things to the last moment." -- The Doctor
"Telescopes are time machines." -- Carl Sagan
I've been trying to plan out my first dom but am totally stuck in indecision! I decided on Plant/Ice a few days ago, but that one's pending creation until I think of a perfect name
After reading a lot here about the new dom changes I decided I definitely want to make a few doms 
So my new idea is a Fire/ dom!
I'm looking for advice on the best secondary for synergy and fun, but am having a bit of trouble drawing conclusions without updated numbers on Mid's. My thoughts so far:
<ul type="square">[*]Electric! Good single target damage... how does this stack up with the dom changes? How is the AoE?[*]Energy! Heard a lot of good stuff about the new power push, and am interested in trying this set out. Also has disorient to stack with Flashfire. Is the lack of AoE frustrating, or do imps fill that hole?[*]Fiery! I heard flares is lacking, but otherwise this set dishes it out. How big a problem is the lack of mitigation?[*]Icy! Looks like a nice blend of ST and AoE with power boost to boot! But is it really end heavy? Weak compared to other sets?[*]Psi! Looks like a potent mix of ST and AoE with the new changes. I've never tried Drain Psyche, but it looks like an amazing power.[*]Thorny! I know nothing about this set :P[/list]
So! I'm looking for a breakdown of the pros/cons of pairing each secondary with Fire/, or just combos that you guys like. If it's not too much to ask, I'm also interested in sets that combo nicely with Plant/ or Earth/, since I've never tried those control sets. Btw, I plan on teaming 90% of the time.
Thanks!