Suzumiya Haruhi no Yuuutsu is revolutionary anime in several ways, so for me it has the upper hand over Black Lagoon which is also a great masterpiece, but in more mundane , complex and adult way. Lots of titles in Spring season, but I can't vouch for the rest.
Ergo Proxy is my favorite anime of the past which is still a full head ahead of most anime, though I see its imperfection nowadays. I also see Fate/stay night and Hellsing Ultimate as minor masterpieces deserving a mention.
Lots of decent works, none is good enough for me. If I really needed to choose, I'd probably go for Hachimitsu to Clover as with all its flaws it can still be nice at times.
Fall has become even more saturated than Spring lately. Beck is something unique that I'm willing to cherish. Genshiken is a cozy place to return to. Zipang is probably the best navy anime, at least to my awareness.
Did not want to give Jungle wa Itsumo Hare nochi Guu Deluxe a masterpiece to give some other decent anime a highlight, but 'tis a barren wasteland this season.