What is the strongest class in Dungeons and Dragons?

Answer 1

It depends on exactly what version of D&D you're playing.

In 3rd or earlier editions of D&D, any caster is going to dominate once you get higher in levels. Fighters and Barbarians will be stronger at the very low levels.

In 4e, there really is no one class that can dominate. Warlords can have a huge impact on the effectiveness of a party, but still needs to use other party members to truly shine. Strikers (Rogues, Sorcerers, Rangers) of course do the most damage and are relatively easy to play, so most people gravitate towards them. But even non striker classes are often only a couple of d6s behind in damage per hit compared to strikers so even there, it's not that great a difference.

If you're talking about the online game, then I hear artificer warforged are pretty nice, being able to do area control, and heal themselves easily.