This is such a hard question. D: There are so many that I really enjoyed. I'm going to have to go with AIR TV, though. It just had a really beautiful story, and honestly I can watch it over and over again. A lot of people don't like it because there is not really much ACTION, but it's just a really great story and it's really touching.
Another favourite would definitely be Fullmetal Alchemist. Again, it's such an amazing story and it really grabs people's attention with all it's action. There are also a lot of characters, so I'm sure that every person can relate to at least one of them. And uh... BLEACH. again, really amazing and so many loveable characters.
I guess in the end I can't pick and ABSOLUTE favourite. XD;