Frankly, all three are good characters, and it really depends on your preferred playstyle. My favorite character of all is my Illusion/Radiation Controller. You can see why in my Ill/Rad guide. He is my most flexible character and can handle most situations with ease. He is great solo, on small teams and on large teams. He is my fastest character for doing Tip Missions because he is so effective at stealthing and distraction -- even the Tip missions that give others problems, he can handle with ease. He is always the first one I pull out for new content. He rarely dies and is always effective.
I also really like Fire/Rad controllers. I leveled one up to 50 with a designed team on a server that is not my main server. I didn't want to move him, so instead I leveled up a second Fire/Rad to 50 on my main server -- still working on his final build. A full team of Fire/Rad controllers can easily steamroll ANYTHING in this game in seconds. But the playstyle is quite different than Ill/Rad -- my Ill/Rad stays at range and relies on distraction and single target damage, and he is better at single hard targets than large numbers of lower level foes. My Fire/Rad is better in melee (with Hot Feet+Choking Cloud) and with larger numbers of lower level foes. The Fire/Rad dies more often, but kills groups faster.
I haven't leveled up a Fire/Rad corrupter yet. I'm working on a Fire/Dark, and that is a pretty awesome combo. But the Corrupter's playstyle is nothing like a controller's playstyle. The focus is on damage over control.
Take a look at my Ill/Rad guide -- the section on Radiation will help you with all three characters.
LOCAL MAN! The most famous hero of all. There are more newspaper stories about me than anyone else. "Local Man wins Medal of Honor." "Local Man opens Animal Shelter." "Local Man Charged with..." (Um, forget about that one.)
Guide Links: Earth/Rad Guide, Illusion/Rad Guide, Electric Control
I just dinged 50 on my ill/rad last night. I'll echo Local Man in that it's probably the most versatile AT you can have in this game. After 4 years + of CoH, that toon is my one and only 50 (no kidding). Of course, a terminal case of altitis is the main culprit for that, but it's saying something for the staying power of that combo (for me anyway). I soloed all the Praetorian story arcs (minus 2 contacts that were "dead" to my toon, and the big SS/Inv elite boss -forget his name- ), I soloed 95% of the CoH story arcs (the ones available post lv 20). With perma-PA your level of awesomeness is just incredible. It's also great on teams too. Another Ill/Rad and I duoed Eochai last night. There was also a scrapper there (I guess he helped as well :-) ). Ill/Rad is totally awesome.
Not to say that the other AT's or controller combinations are not, but in so far as true power, I don't think anything can top a well built ill/rad. My 2 inf.
Ok, since returning to the game I have had a terrible case of altitis. I currently am stuck between an Ill/Rad, Fire/Rad corruptor and a Fire/Rad controller.
I just got the Fire/Rad controller to 20 and got stamina which helps a ton but so far he's been a bit painful to level up. I guess I'm wondering for damage wise which will be better to stick with in later levels.
All toons are about the same level with the ill/rad in the 20's. Any thoughts or comments?