Mm, I have pretty varying taste in manga/anime. I think I like a series as long as the plot is interesting, there are a couple characters that are interesting or that I like, and it gives me something to think about. I tend to like more seinen, psychological, historical, or steampunk type manga/anime, but I'll try anything once and there are a bunch of shoujo, shounen, yaoi, and yuri I like too. Humor is also a definite plus but only if it isn't corny/cliche/just plain stupid.
Let's see.. My "All-Time Favorites" list is
--Revolutionary Girl Utena (anime series, NOT the movie; I haven't read the manga yet.)
--Planetes (I like the anime better.)
--Death Note (Both the manga and anime are totally awesome.)
--Ergo Proxy (The ending was... :? but I really loved the series.)
--Hellsing (manga and OVA. The anime was good for the music. :roll: )
--Code Geass (I haven't read the manga...)
--Ghost in the Shell: SAC (both seasons of the series and Solid State Society)
--Evangelion (haven't read the manga...)
--Sex Pistols (licensed under the title "Love Pistols" in the States)
--Fullmetal Alchemist (manga. The anime was OK. :roll: )
--Gintama (I haven't read the manga yet.)
And I've started reading Welcome to the NHK. It's pretty awesome and I think it'll probably end up on this list too.