Out of all of the 3DS launch games Super Street Fighter IV 3D Edition has received the best reviews. What is also interesting about it is that it's a lot easier to play over past Street Fighter games because of the ability to map special attacks to the touch screen. This could make the game a whole lot more appealing if you aren't a hardcore fighter fan. It seems like a reliable choice.
I have Lego Star Wars III for the Wii and I can confirm that it is quite an enjoyable game. Maybe it's not the best Lego game, but it's definitely up there. I've heard that LSWIII translates pretty well to the 3DS, but I can't confirm that it's the same experience as it is on the consoles.
I would also urge you to consider games like Pilotwings Resort and Steel Diver. While they haven't received quite as good reviews, they look like quite fun and enjoyable games. Maybe not worth a full 40 dollars however.