I am a sucker for bots:
Bots/Poison - ever elusive
Bots/Dark - stuntastic
Bots/TA - surprisingly solid
but my favorite is Bots/Storm!
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
Bats/Tarps all the way, though my Thugs/Pain is a close second.
Right now for me it's a tie between Thugs/Pain and Necro/Dark. Both sets give you something to do all the time and they cut through spawns at breakneck speeds.
Worst Mastermind IMO is Bots/FF. You stand around with nothing to do a lot, and Bots kill slowly compared to any other primary (save Mercs).
The best comics are still 10�!
My City of Heroes Blog Freedom Feature Article: "Going Rageless?"
If you only read one guide this year, make it this one.
Super Reflexes: the Golden Fox of power sets!
WARNING: I bold names.
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Bats/Tarps all the way, though my Thugs/Pain is a close second.
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When did we get bat summoning?
"The danger is not that a particular class is unfit to govern. Every class is unfit to govern." Lord Acton
Madam Enigma's History
Necro/Poison-Most recent MM and a blast to solo.
Necro/Dark-Fun to play and tears through mobs.
Thugs/Storm-Awesome, need I say more?
MMs in general are awesome for me.
Probably not a hole lot of people will agree with me but i love my ninja/dark. That was the only mastermind i could play. Now i am trying out a nec/pain and like it but it always leaves me wanting to play my Ninja/Dark.
[url="http://teenage-kid.deviantart.com/"]My Da page![/url]
Virtue
Ninja-Bee- 50 Ninja/Dark MM
Aqurian Draconum- 50 Archery/Fire Blaster
Korrawi- 50 Super Strength/Invulnerability Tank Plus many others!
I'm really enjoying my Necro/Pain right now - I find I can support a team while doing excellent damage with my practically-unkillable pets.
Close second is Ninjas/FF. I mean, really, ninjas with force fields. How freakin' awesome is that.
@Amarlex in game, on the Virtue server.
Bots/traps, so many fun toys
Bots/Traps, even at 50th level.
Although I AM enjoying my Bots/Storm.
Thinkin about a */Pain, any suggestions?
Thugs/ or Necro/. Dunno exactly why, but Thugs/Pain seems to be quite the steamroller. Necro/Pain makes your already fairly-tough pets into gods.
@Amarlex in game, on the Virtue server.
Thugs and Necro all the way for me. Necro/Dark was my first toon on villains (and first villain to 50) and will always hold a special place in my heart. 
-Stygian
Bots/FF. Untouchable and full of awesome sauce.
My Ninja/TA probably amuses me the most, but I'd probably rate my Necro/Dark as my favourite - not far behind in amusement stakes but also feels a fair bit more powerful.
Although my Bots/FF is probably the most invincible, she's also really dull to play.
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Bots/FF. Untouchable and full of awesome sauce.
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I actually have to go with this. It was my first MM, too, so perhaps it's only sentimental.
After that, we'll say Thugs/Poison.
Where to now?
Check out all my guides and fiction pieces on my blog.
The MFing Warshade | The Last Rule of Tanking | The Got Dam Mastermind
Everything Dark Armor | The Softcap
don'T attempt to read tHis mEssaGe, And believe Me, it is not a codE.
may favriote..hummm.. mine lol...... so far id say my thugs/dark
Bots/FF was my first MM when CoV came out. it took several false starts to get going with him tho. Now he's 45. Pretty damn solid with IO sets. I have several other MM combos I'm trying out. My favorite and most fun so far is my Necro/Dark that just hit 50 today. Just all-round fun to play, especially on teams. 
50 Inv/SS Tank, 50 Inv/EM Tank, 50 FA/SS Tank, 50 Shield/SS Tank, 50 WP/SS Tank, 50 Dark/Dark Tank, 50 EM/Elec Brute, 50 SS/FA Brute, 50 SS/SR Brute, 50 Fire/Kin Controller, 50 Plant/Storm Controller, 50 Earth/Therm Controller, 50 Necro/Dark MM, 50 Bots/FF MM, 50 Elec/SD Scrapper, 50 Arch/MM Blaster, 50 Emp/Psi Defender
Hmm tough call, I got 6 MMs, but my fave has to be my Thugs/Pain, Bloody Red Rose. I must've started her over a dozen (or MORE) times, but she is PERFECT now. She's att 44, I'm sure she be my first 50. I've yet to see a EB or AV she can't take down, the Pain takes the best parts of Thugs and cranks it to 11. You'd think my fave would be my Thugs/Storm, all those killer debuffs ... but I just plain have more fun with Pain. Go figure.
Shigeru Miyamoto "A delayed game will eventually be good, a bad game is bad forever."
I think my favorite concept I have is a Natural Thugs/Poison. As a bonus, he's VERY fun to play.
Story goes he's the common tough guy with a following of thugs. He started smoking at 9, and somewhere around 20 switched to cigars and hasn't been seen without one in his mouth since. Sure enough lung cancer comes along, but the bot/ff technology MM character develops something to save his life from the lung cancer but also managed to harness the cancerous byproducts as weapons.
Lung cancer: Not without its benefits.*
*Kudos if you've seen the movie that line is from.
Where to now?
Check out all my guides and fiction pieces on my blog.
The MFing Warshade | The Last Rule of Tanking | The Got Dam Mastermind
Everything Dark Armor | The Softcap
don'T attempt to read tHis mEssaGe, And believe Me, it is not a codE.
Solo?...Necro/Dark
For teams?..Thugs/Pain
SHORT ANSWER:
Primary: Thugs
Secondary: Traps or Storm
GET OFF MY LAWN (RAMBLING) ANSWER:
It is hard for me to be certain because a hardcore push into Masterminds is only a recent thing for me, say within the last year. I've had some masterminds plinking around at various low levels since they came out but only in the last year have I invested any time.
As beautiful as Robots are, I think I like Thugs more. I have a level 48 Thugs/Dark and he is great. I have a level 34 Robot/Traps and I LOVE LOVE LOVE him. I think I am liking Traps more than I guessed I would. I am glad I never had Poison Trap back when it was uber though. Anyway, so I SUSPECT my Thugs/Traps I have been playing recently is going to become my favorite by the time I get into my 30s on him. I've also enjoyed several /Storm Masterminds, and a couple of /Poison masterminds.
Here is what I am thinking for my favorites:
BEST FAVES:
Thugs/Traps
Thugs/Storm
Robot/Traps
Robot/Storm
LIKE A LOT:
Necro/Storm
Necro/Poison
Thugs/Dark
Thugs/Poison
LIKE MORE THAN I EXPECTED:
Mercs/Storm
Thugs/Pain
FOUND TO BE LACKING:
Ninja/Dark
Anything/Trick Arrow
Anything/Force Field
UNCERTAIN:
Ninja/Storm (too low too tell yet, honestly, just a feeling)
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
Merc/Storm is the only lvl 50 MM that I would keep playing. A lot of 50 toons just retire once they hit 50.
I used to like my lvl 46 Necro/Storm a lot because I built him with tohit debuff sets in Grave Knights and Lich so they can shoot dark blast and gloom faster but the dev nerfed that. I haven't gone back to him at all after the nerf. This could be my #2 most favorite.
I find my lvl 50 Merc/Poison quite lacking on a large team. I don't enjoy him as much as I spend way more time resummoning.
What's left is to normalize all Assassin Strikes and improve Stalker's old sets (Claw, MA and EM)! You don't need to bring back the missing PbAoE attack. You just need to make the existing ones better! For example, make Slice a WIDER and LONGER cone.
Thugs/Poison, inevitably I suppose, since Nightshade was my first, and to date only, 50. I don't like to stand around a lot leaving the pets to do all the work, so the very active /Poison powers are great (plus dual pistols for added coolness)
Also, Thugs is a very visual set , with a lot of stylish, clever or just plan funny touches - the sideways aiming of the punks, the arsonist's zippo-fuelled fire breath, the bruiser riding in on his harley, the enforcers just looking utterly bad-[censored] once they get the second upgrade... and I sometimes miss the slapstick of the bruiser charging into a crowd of Longbow and unleashing handclap like a big mean bowling ball. If I didn't have a full IO set in there I'd be tempted to slot a KB enhancer, just for the lulz
I've played a couple of other sets, and have a Mercs/Dark in the mid-teens who seems very powerful (so much so I can't help feeling it must get really bad later on) but Thugs/Poison is probably the most out and out entertaining powersets I've played, on any AT.
I'll have to go with my first red side 50, my Thugs/FF, not only does he steamroll everything(Except Statesman and Natzi Statesman) Nothing like not having to worry about a constantly dead Pyro with a bubble around him, burning everything around. I thought FF would be boring, but I notice with my abilitly to knock things down and then my pets working, with tripple stacked leadership and masive bubbles, this is really fun.
My Necro/Dark is probably more useful on teams and definitely a powerhouse, but my Bots/Traps is more fun by far. Of all my MMs my Mercs/Storm I'm leveling up is the only thing that comes close.
So what's your favorite MM you've ever played?
Could it be Zombies/Dark? Possibly Ninjas/Storm? How about the ever elusive Bots/Poison? Maybe the wonderful Mercs/TA? Oh, wait, I know! You gotta be loving Thugs/Traps, right?
My personal favorite recently has been Zombies/Traps (Admittedly), but in the past I enjoyed Zombies/Dark. What can I say? I'm a zombie fanatic.
But how about you? I'm interested in seeing what your favorite Mastermind is.