Magic Magnum said
See's constant mention of Digimon.It's Intro is just the best, and honestly I liked it even more than pokemon.Digimon show = Far better than pokemon.Digimon games = Just as good, but sadly didn't seem to get close to as popular.
Honestly I think the Pokemon games beat out the Digimon ones, mostly in terms of consistency.
In Digimon, all the rules change with every game. I only played them casually and I didn't even pick up that many, but the gameplay and, in particular, the digivolution paths were all so different that it made it difficult for me to really think of the games as a series, or transfer any useful knowledge from one to the other.
In Pokemon, X evolves into Y (no pun intended) and that's very unlikely to change. Every species has specific characteristics and those don't change... much, other than adding new attacks and maybe a new evolution. Not to mention most of the items don't change, either - nor do established attacks. There's not nearly as much re-learning required as in Digimon. If you've played one Pokemon game, then the skills you learned there will transfer into the next game you play. And, virtually all the gameplay mechanics should remain the same, as well. That's probably why the Pokemon games did so much better than the Digimon games, now that I think about it.
Edit: But yes, that opening theme is the best thing ever.
You would think that the repetitiveness and the lack of variation in lyrics would be a negative factor, but somehow that seems to be the best part.