Most of all, Kid Icarus: Uprising and Mario Kart 7. If you're cool with remakes, Ocarina of Time and Star Fox 64 are worth getting too; besides that, Sticker Star, New Super Mario Bros. 2, and 3D Land are all good games, but if you're iffy on them probably better off passing or waiting for a price drop. Mostly since the latter two are pretty shallow games that have a good bit of padding to make up for being in themselves pretty short and lacking in content, while the former is just weird and has a lot of baffling, counter-intuitive design choices. A big one probably being that 99% of the fights in it are completely meaningless and not worth bothering to do anything but run from.
Like most others I'd go for Mario 3D Land, it wasn't until I replaced it that I realised just how good it is. NSMB2 on the other hand isn't worth the price it's currently at. In the unlikely event you haven't played OoT before then just buy that.
The eshop has some good titles too, VVVVV and Pushmo/Pullblox arguably being the highlights.
Personally I don't see the point of screen protectors for handheld with a clamshell design that comes with some padding to put in between the screens when not in use.
Patti Mayonnaise Captain Pixel: the Game Master