I've heard that OoT has really good 3D. I'm picking up a 3DS XL Friday and I've never seen one played before, I want some games that will really impress me with the 3D. Of course, that's not all I'm concerned about, I want good gameplay too, but that's not what this topic is about.
I plan on getting Mario Kart 7 (assuming I can find the bundle), OoT, Luigi's Mansion, and Star Fox 64 as my free game. I don't know how good the 3D is in those games, so feel free to let me know, but I am getting them regardless. I'm also looking into Sticker Star and Fire Emblem in the future.
Ocarina of time definitely has a powerful 3d effect as well as Resident Evil: Revelations. Kid Icarus is nice and games like Mario 3d land and fire emblem aren't over powered but just make the games look so much better.
Luigi's Mansion: Dark Moon has really great 3D. That was my first game and it really blew me away. I've been playing the heck out of Ocarina of time lately and that also has really very impressive 3D. The thing is, the game's look awesome WITH the 3D and are still really pretty great looking with it off also.
(b^_^)b Snake "A strong man doesn't need to read the future. He makes his own."