Freedom Force. God, I loved that game.
If you've never heard of it, gameplay-wise, it's basically the precursor to the Marvel Ultimate Alliance series, as in it's a top-down squad-based RPG.
It's a barrel of fun, it's got this whole golden age of comics feel to it, with an over-the-top announcer who narrates the story, and villains with comically silly plans. For example, there's an evil alien force that wants to take over the world, and they decide to do this by sending canisters of purple space radiation to the planet, in the hopes that the most evil people in the world will gain superpowers, and destroy the world. This, of course, backfires in spectacular fashion, but this isn't the game to turn to if you want logical motivations or decisions.
The heroes themselves are fun to watch, but instead of describing them, here's a picture:
Possibly the best part of the game is that it comes with a hero creator, and it lets you import your created heroes into the single-player game to use.
And if you aren't conviced yet, it's made by Irrational Games, yup, the same guys that made Bioshock. It's on steam right now, check it out, you won't be dissapointed.
