"Most powerful class" needs a bit of discussion. Best core class if you take it to level 20? Best low level class? Best dip class? Best class to serve as a base for PrCs?
Druid and monk are probably the only classes interesting up to 20th level themselves, though some may also include barbarians and rangers. Other classes, like the cleric, wizard, or sorcerer, get so little that a pure build would be boring and not "the best class". But cleric seems to be a popular option, so is "best" more of a "class-base", even if other PrCs are intended?
I've yet to hear anyoen want ot play a fighter 20, but as a quick dip for extra feats I'll often see a bit of fighter for a non-spellcasters. Same goes for rangers, especially first level for skills.
"Best" is to vague - each has their strengths, depending where and how you use it.
Cheers,
=Blue