Destil
Explorer
Unless I've missed something radiant has the worst energy kicker feat (it's nice but epic level and only on a crit). While there's some good general feat support for it in Divine Power, nothing you can do with it is on par with Psychic Lock or Resounding Thunder. Fire likewise isn't that great (no ongoing damage on a crit feat? Really?) but it's pretty easy to burn through resistance with it. Psychic is most likely the strongest overall, being rarely resisted and having a fantastic support feat.Various damage types have their various niches. Some are overall better (radiant) or worse (poison) in the sense that they are more or less resisted by monsters in general.
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There are specific feat paths that work well with specific damage types too, like frostcheese (Wintertouched and Lasting Frost). Its hard to say which types overall are "best". Radiant in isolation is probably better than the others, but remember that radiant powers take this into account (or should at least) and are presumably overall a bit weaker.