Perfect Dark is Goldeneye with aliens and more stuff, so just as good, just bigger. I highly recommend it.

Donkey Kong 64 isn't bad for multiplayer, and the same goes for Conker's Bad Fur Day (which is seriously awesome in it's own right).

Super Smash Brawls is definitely needed as well.

[strike]It would certainly be worth investing in each and every one of the Mario Party games on the N64.[/strike]