I'm more into the fluffy BL than the full-blown yaoifests, so my favorites are Gravitation, Legal Drug, Gravitation, Loveless, Gravitation, La Esperanca, and Gravitation. Sometimes I like Gravitation too. XD
Okay, anyone who knows anything about me knows I'm totally obsessed with Gravitation... seriously, don't even get me started. Just leave it with a nice lil' "kill me shining". I don't think anyone's list is complete without Gravi, really.
Legal Drug... Clamp. Pretty boys. Put two and two together and you've got one of my favorite manga of all time. I just wish Clamp would continue it, though... from what I hear, it's been postponed. -_-
Loveless is really sweet, angsty, action-packed, and mysterious all at once. Even if it wasn't shounen-ai it'd still be a great series, it just wouldn't draw as many fans. Oh, and it has cat ears. Did I mention that?
La Esperanca is really cute. I like the art styles in it. I only have the first volume so far so I can't say much about it, but I like what I've seen... is it worth it to get volume 2, does anyone know?