The two games that I have for the 3DS are both pretty amazing. I'm really enjoying both of them.
But I have to say, "Street Fighter" takes the cake. It's simply amazing to look at, controls like a dream, and is an absolute blast.
I'm normally not very good at fighting games, but this one has taken some steps to make it a bit more accessible. Mainly, you can set the touch screen as "buttons" and map the difficult to do special moves to a single press on the screen. It makes it so that a guy like me that doesn't really "get" fighting games can in fact pull off some of the awesome moves and have a good time.
"Steel Diver" is no slouch, though. It's unique and also very fun to play. I'd say my only problem with it is going to be the fact that it's only got about 7 levels. They're awesome, but that's just not enough. Hopefully a sequel will make its way onto the system.
Overall, though, I'm REALLY impressed by this new piece of hardware. It's really cool.