For solo, I prefer to steer away from a secondary that has DoT damage, as it will break the sleep effects of two of Mind's low level powers, Mesmerize and Mass Hypnosis. Even then, Fire can work around those problems with smart target selection and use of other mezzes.
Psi is a great secondary still, despite the reduction (which was bound to happen) to Psychic Shockwave, and the Mind/Psi combo will only really have issues against Psi resistant enemies (though there seem to be a lot at the higher spectrum of the game), but even then, if played well, it'll just mean it'll take a noticeable longer time defeating enemies.
Energy has gotten a nice boost with the Dom upgrades, and has the easiest great single target ranged attack chain for Dom secondaries. Power Push is now a premiere power for the set, especially considering its amazing base tohit. That's what allows Energy to have such a good ranged attack chain. It also gets Power Boost, which goes well with Dom primaries. Its one downside is the need to control KB, which is still quite doable, especially solo, and really a very small downside for what you're getting.
Electricity is also a very nice set with very nice cast times, and the only secondary that has a full-out Build Up (Fire has Fiery Embrace, best paired with its Fire attacks, but not as good outside of it and with a longer timer, and Thorns has Aim), which does have a great impact over time when used often, considering the upgraded base damage values of Doms.
At that point, it's up to you to decide. I'm sure others will give preferences and advice.
If relying on ST damage. Energy and Electric.
However with Fire/Thorns/Energy, everything is dead to fast to care. I fine Mind/Thorns just awesome and AIM + Domination turns mind into a monster of a primary. I work around the DOT issues with thorns by being careful not to use any Cones until necessary (if I'm relying on sleeps and soloing). Otherwise I just go for it.
My level 50 Dominators:
Madame Mindbender 50 Mind/Energy
Fly Agaric 50 Plant/Thorn
Nate Nitro 50 Fire/Psi
As previously noted, ultimately almost anything will work well with mind. The best synergy is probably from sets with more of a single target focus - energy and electric are going to give you your best single target damage, with electric winning on burst damage (due to build up) and energy on sustained damage as it has a large number of high damage attacks. I am not sure that the set's with DoT's are as bad as they would initially sound, at least for single target attacks. The primary controls in mind are really dominate + confuse, with the sleeps as backup or area controls so if you focus on single target damage a dot is not going to break those controls - its AE where DoT's will mostly mess up mind as your AE controls are either on long timers (Mass confusion and total domination) or are easily broken (mass hypnosis and terrify).
Pro's of each set
Electric: Mostly energy damage, build up, early on you can choose to focus on ranged or melee and have a good attack chain
Energy: Lots of high damage attacks, good extra control (stuns + KB and power boost), can be either melee or ranged focus as well but takes longer than electric
Fiery assault: Unresisted damage type, good utility (fiery embrace for damage + consume for end), fast high damage ranged attack chain once you get blaze, best AE in secondaries
Icy: Slows + power boost
Psi: Slows + good utility (drain psyche is pretty awesome), second best AE in secondaries (just after fire) and could be considered best due to fast activation on PSW.
Thorny assault: -def on all attacks
Con's of each set:
Electric: No good tier 3 blast (you get a questionable pet instead), tier 3 melee (thunderstrike) only matches energy with build up, limited AE
Energy: Even less AE than electric, also very slot expensive compared to electric as you have to take and slot up more attacks to compensate for not having build up.
Fiery: DoT's will interfere with AE control, ranged attack chain incomplete until L38
Icy: not spectacular in any one area (ok single target, ok AE, no real cool tricks other than slows), ranged attack chain incomplete until 38
Psionic: Late game enemies are not only frequently resistant to psionic damage but many also have DEF to psionic, which makes having all psi damage questionable.
Thorny Assault: To many dot's plus far to many slow activating attacks, a lot of AE but overall AE damage it not at the top, utility powers (thorntrops) are going to break a lot of minds AE controls.
One note - Fiery Embrace vs Build up. Its easy to dismiss fiery embrace due to the long recharge - however if you look at the actual boost it gives and uptime vs downtime it is probably the best native damage boost that dominators get. Build up is nice - but it is up for 10 seconds out of 45 (22% uptime) with fully slotted recharge reduction and it only gives a 68% damage boost. FE can be up for 30 seconds out of 90 with fully slotted recharge reduction (33% of the time) and gives an 85% damage boost to fire attacks. Now the to hit boost in build up tends to balance them out but if you are looking for pure damage FE is a great power, much better for dom's than it is for melee builds as they get it in a defensive set and will only get the max boost from it if they take fire melee as their offensive set.
EDIT: Correction - FE gives an 85% damage boost to fire powers, not 80%, misremembered the value.
Globals: @Midnight Mystique/@Magik13
Hi folks! Back again for more of your sage wisdom!
I'm in the planning stages of a new MC Dominator, and I was wondering which secondary you thought was best to good for soloing it. (I mainly don't want to gimp myself!)
Really all of the secondarys look interesting, and they'd all fit my theme well, aside from /Ice and /thorns, and probably not the upcoming /Earth. So, I guess my questions geared more towards Electric, Energy, Fire, and Psi... I do really like Psi...but would really like to try Electric or Energy I think...
Thanks for any advice you can give!