Personally a nice combo to level would be Earth/Fire. Earth comes with a lot of control and fire comes with plenty of damage, seems like a nice combination.
Not really a fan of Psi, but I would go with Fire if I had to choose. Fire comes with decent enough Control lots of damage with the Imps, as far as Psi, I didn't take Stamina due to drain psyche, and I heard TK thrust got a bit of a buff.
Lastly, I would go with Plant/Psi, Plant is real nice to have, Domination + Seeds of Confusion is just awsomesause, Creeper Vines with your AoE immob, Plant is another control heavy set that is nice to have, personally I would probably try to pair with /Fire but not sure exactly what you are looking for.
"Yes, winning all the time can be boring."
-Knight_Chill
"It's amazing how well you can put up with endurance issues if you hide them under a large enough pile of bodies."
-Spiritchaser speaking on Dom Revamp
What was your first Dom? I mean primary and secondary?
Anyway, of the four you mentioned, Fire/Psi is really good (though not as glorious as it used to be) but I think it could be a bit of a long haul even after the adjustments (in which the top attack was nerfed but some of the others are improved). I played one to level 50 myself and I really like it (and I never even had Perma-Dom on mine).
Plant/Fire is also really great. That was my first Dom and its pretty easy and straightforward.
I also found Earth/Fire to be really cool as well, my Earth/Fire is level 43 or so.
One combo that I like, that most people may not, is Earth/Energy. I know that Earth is already just about control overkill, and that Power Boost from /Energy makes it even more overkill. I also know that the blasts can knock stuff out of your location-based controls. I also know that /Energy is not an AOE set by any standard. I still found that I really love the combo.
Fire/Earth was nice and meaty. Lots of smash, lots of playing in close, which works well with Fire Control. Its a crunchy combo.
Finally, one set I don't recommend because its a serious pain in the butt, but its nice now that I'm finally high level with it, and that is Gravity/Fire. The control it has comes late, and its a bit of a chore, but I kinda like it because it's NOT sexy. 
If that makes any sense.
Good luck!
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'm goofing with a fire/fire and I like him enough that I will delete him and he will be my first GR praetorian. Very fun, highly recommended.
I have an earth/thorn that is ok. Earth/ is fine and fun but /Thorn is way too slow for my taste. I just use her as a market bot now.
Both Earth/ and /Earth are cool sets with both great powers from a game standpoint, and very nifty graphical options from a visual standpoint.
"Hmm, I guess I'm not as omniscient as I thought" -Gavin Runeblade.
I can be found, outside of paragon city here.
Thank you everyone at Paragon and on Virtue. When the lights go out in November, you'll find me on Razor Bunny.
I want to start another dom since I got my first dom to lvl 50, which was my first 50 aswell. I was thinking Fire/Psi, Plant/Fire, Plant/Psi, or Mind/Elec. Just some Pro's and Con's would be great.
Lyow-BS/Regen-50
Time Flawed-Grav/Eng-50
Malicious Despot-Crab Spider-50
Nictuz-WarShade