Well firstly, if you don't already have a DS, remember you can play DS games on a 3DS, such as Pokemon Black and White.
The remake of Ocarina of Time is pretty good. Especially since they tweaked the interface so switching boots in the Water Temple only takes a button press rather than rummaging through your inventory every single time. There are graphical improvements but they haven't ruined it.
The game I've clocked up most time on is Street Fighter 4. It's smooth and playable, it's easy to enjoy for five minutes during break/lunch time and there is are optional 'noob' controls that let you activate special moves (including Ultras and Supers) via the touchscreen. Even against the AI it is a lot of fun.
I've also enjoyed the New Super Mario Bros 1+2 and SMB 3D Land. The games do make it hard to lose by allowing you to easily farm early levels for extrs lives, and they are also a little slower than the originals. But they do introduce the challenge of collecting big coins which in many cases are interesting puzzles (although most aren't hidden or tricky at all).
The virtual console offers a number of old classics from the NES, Game Boy and Game Gear (which was a SEGA handheld) but mostly arcadey games not RPGs. They are from the era that spawned the phrase 'Nintendo hard'. Ninja Gaiden, Ghosts and Goblins, Mario Lost Levels, Punch Out, etc. Oh, and Legend of Zelda and Zelda 2: Adventure of Link. But the selection isn't that big and they haven't got a SNES or Megadrive section.
If you've got a weakness for platform games look for the Sonic Classic Collection, with Sonic 1,2,3 and Sonic & Knuckles.
