Ocarina of Time is probably the largest/longest game on the VC right now and I can't recommend it highly enough if that's your style of game (Adventure/Exploring/Puzzle Solving.) but, like most N64 games, the graphics haven't stood the test of time, and can be some what distracting. If you have a high tolerance for triangle faces though, this is definitely a game that should be in every zelda fanatics library.
Another good and long game on the VC is Breath of Fire II. It's a turnbased RPG like the old Final Fantasy series, but I prefer it to the FF games. Something about turning into a dragon in combat, I dunno, it appeals to me. ;P
*Also* If you want to keep yourself occupied for a while, you can get Zelda I and II on the NES for the same price as Ocarina of Time. Just something to think about.
There's a bunch of great shooters on the VC, like Contra and R-Type, that are pick up and play games. I.E. you can play through them a bunch of times with out getting bored.