Even though I prefer the manga, I'd have to pick Bleach as my favorite 2004 anime.
It was pretty popular at its peak, so I imagine most people here have heard of it...but for those who haven't, it's basically about a high-school boy that can see, and eventually learns to exorcise, wandering spirits. It's largely a battle manga that follows several shonen tropes, but even so, the fights themselves were usually fun to watch. The main cast is pretty great, with the humans and shinigami having several extremely interesting characters, but it was the eventual introduction of the Arrancar characters that stole the show for me. Despite being antagonists, I found myself wanting to know more about every one of them, and even found myself rooting for a few. Overall, I'd say my personal favorites (of the whole cast) are Ishida, Orihime, Kenpachi, Grimmjow, and Ulquiorra.
On the downside though, Bleach has to be one of the anime with the most absolutely massive difference in quality between canon and filler episodes. This is definitely a complaint shared with most long running shonen adaptations...but holy crap the filler episodes are terrible. Everything feels so forced, and there are even times when it flat out contradicts what the canon material has established. Even though I recommend the series, I'd have to recommend finding a canon/filler guide to go along with it...just to avoid the dreck that drags the rest of the series down.
Back to a more positive note, the OST is absolutely incredible...and I still find myself listening to it on the way to work every now and then.
NNID: Zephyr25 / PSN: Zephyr--25 / Switch: SW-4450-3680-7334
