One Piece. All One Piece, all the time.
... Well, here are some honorable mentions I guess: Fairy Tail, Hunter x Hunter, Ouran High School Host Club, Ranma 1/2, Seven Deadly Sins (one of the few recent series I've been able to get in to), FMA, Utena(even if it is repetitive and the movie is weird, still brilliant).
I feel like my list is deceptively heavy on the shounen series, but that is mostly because I like to get invested in long series I can watch over over time. 12-26 episodes is just never enough. Ie: I loved Honey and Clover, Chobits (really anything by Clamp), Midori no Hibi (never understood why it wasn't more popular), and a ton of eclectic stuff, it is just all so fleeting it falls to the wayside long term unfortunately.