As it would be impossible to pick an absolute fav, I'll just list a few that stay, or will stay in my collection forever.
Beck (This show made me pick up my guitar again, now that's quality anime! And I can sing the intro and not get funny looks.)
Cowboy Bebop (Saw it first on Adult Swim back in '01 Got me through the x-mass season working retail at the mall...)
Escaflowne (First show to ever sit me down yearning for the next episode to finish, back on kazaa days before BT)
Gantz (After each episode I hunger for the next, sci-fi, human, and intense! My stomach gets all in knots and the episodes just scream by way to fast, awesome!)
Ghost in the Shell: SAC (I hated the original movie, and instantly fell in love with the show, now that tells you something!)
Jungle Wa Itsumo Hale Nochi Guu! (whaaa!)
Kenran Butoh Sai (Fun, easy to follow sci-fi, with Pirates!)
Love Hina (I was sat down one day, before my Anime addiction and made to watch it beginning to end. I still have yet to find it's equal.)
Monster (Wonderful, simply wonderful! The coolest most instantly likeable main character I have ever seen in any show. I can't wait to see how Dr. Temma resolves the story.)
Otogizoushi (What a cool and refreshing style! Great story telling and a main character that realistically grows more mature each episode. I love it!)
Samurai Champloo (heh. These guys are so funny it hurts!)
And SunaBouzu is freaking awesome so far, but it's still too early to make any final calls.
There's just no way to pick just one!