I think for me the best, most flexible mastermind I have leveled, to 50 even, is my Bots/storm, but I just love chaos.
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.
After playing Masterminds for the last few years I have looked for the very best set. The Demons had a high resistance to damage and Thermal increased it even more, but it didn't seem to matter as they drop in health anyway due to no Defense. I thought of using traps or FF to alter this, but then no heals for the damage they do take. Aid other is too clumsy and sluggish. In all the secondary sets available to demons I have yet to see one that worked well.
The best MM set I have played to date was Bots/Thermal for the Defense, Resistance, heals, debuffs and buffs. You lose the 103.25% damage buff from Hell on Earth (HoE), Forge and Leadership's Assault, but still get 66% without the HoE. This set has successfully soloed both AV's and Heroes since level 35 (as early as I tried). Later on using Melt Armor and Heat Exhaustion, you trash any AV.
The Demons seem to do more damage than the bots. This is why I was looking for an answer for them.