Geez I can't believe this but no one has said Pokemon! He he Just Kidding.. But seriously.. 
I actually really enjoy the Pokémon series. I'm not a fan of the whole franchise (the cartoon really bugs me), but I think the games are great. It's one of the best handheld RPG's I've played. It provides hours of fun and loads of replayability. Great game for obsessive completists. Pokémon Stadium doesn't offer a whole lot, but it's fun if you have a friend. I've only played the original Japanese version that wasn't released in the States (cost me $80 for the game, Game Boy attachment, and N64 mod), so newer versions may have more features. Pokémon Snap was a little ridiculous, but I have to admit that it was tons of fun, although a bit too easy. Pokémon Puzzle League is easily one of the best games I've ever played; even better than the original Tetris Attack. It's a very enjoyable game and easy to get hooked on. Even the Trading Card Game is a lot of fun.
The game may have been overhyped a bit and the market was horribly flooded with merchandise, but it's with good reason. This is one franchise that can definitely hold its own and live up to a lot of its hype. Nintendo knows a good thing when they see it.
And while we're on the subject...Dragon Quest is another great series, and VIII is one of funnest games I've played. It gets pretty hard towards the end, but there's a real feeling of satisfaction once you've completed it. And the Alchemy system is fun to play around with.
Since I'm still talking about RPG's, I should mention that Grandia II for Dreamcast is one of the best ones I've played, and it's one of the few that I actually enjoy going back to every now and then. Suikoden II is another fun one and although it's been out for awhile now, I can't wait to finally play Suikoden III.
