I have Wario Land II DX, that game is pretty awesome even although it is quite easy due to the "no die" system. Wario Land III has been on my want list for a while now... perhaps I should bump that one up closer to the top.
My cousin gave me Pokémon Gold because he didn't like it, and I wouldn't have ever owned it otherwise. It's a decent game but two things which really drag it down for me are the level grinding and not being able to collect every Pokémon without trading with other versions. Revisiting the world of the first game felt tacked-on due to so little events taking place there and the general lack of direction at that point.
Despite having few N64 releases the year 2000 was a great year for the system. My runner-up for that year would probably be ExciteBike 64, as that game is all kinds of awesome. It has a great mix of indoor and outdoor tracks, a few minigames to spice things up, a rather extensive track creator, and even the original ExciteBike but where the track save feature actually works.
Banjo-Tooie is another release which I believe doesn't get enough praise. The huge and detailed worlds which Rare managed to create without the Expansion Pak are truly breathtaking, and the variety of the game is simply staggering. The focus on collecting has been toned down, making it a much more enjoyable game than its predecessor.
I had some good fun with Mario Tennis as well. It may not have much in the way of content, but the gameplay is solid, the character roster includes both recent favourites and blasts from the past, and the multiplayer held its own amoungst titles like Mario Kart and Mario Party.
There are many other good titles as well, such as Paper Mario, Pokémon Puzzle League, Mega Man 64, Kirby 64... Awesome year!