"What superhero RPG have you enjoyed most? Why?"
I...have not played a superhero RPG.
Like, ever. I remember playing HeroClix once, and being terrible at it. I suppose I'm just not really interested in the genre. I appreciate comic books for their culture, but don't tend to read or collect them. Superhero movies are the closest I seem to get to the culture, well, that and the various amazing superhero videogames that have been released, and talking about those here feels like cheating, kind of. But seriously, Marvel Ultimate Alliance is awesome, as is Spiderman 2. And inFamous, actually.
To be honest, if I were to play a superhero RPG, I think I'd want it to be in a setting similar to the one used for inFamous. Opportunities for power-trips, but still vulnerable as a squishy human. Except when you can drop from tall buildings. Gritty, I suppose? I guess I have a hard time seeing how to limit a group of players who want to be supermen. In pre-written media you've got your hero weaknesses and vulnerabilities all worked out beforehand, but in RPGs you have CharGen, and anyone living right now has spent at least a year of their lives, collectively, figuring out exactly how they'd be an unstoppable superhero if it were possible.
The genre just seems to intentionally lend itself to min-maxing. Perhaps the existing systems can pull it off well, but like I said, it's not a genre I have interest in investing time in.