I don't really have a favorite ANYTHING, but as far as manga goes, Hourou Musuko has continuously supplied me with I KNOW THAT FEEL moments and bitch tears flowing down my cheeks for the entirety of it's run (or rather, the entirety of my following it, seeing as it's been going for a really long time as of now).
Really, though, this is a vague question. It's pretty much impossible to just summarize up "manga", expect us to let everything inherent in all of Japanese comics commingle, and have some favorite arise. I mean, with SoL yonkoma stuff, things like Yotsuba and Hidamari Sketch come to mind, but on the other end of the spectrum you can see violent action type stuff, and all of a sudden I'm thinking of Berserk and Deadman Wonderland and the like. Drama manga brings to mind completely different things than Shonen material, and how on earth am I to seriously compare a Mahou Shoujo and a gritty story about life after the apocalypse? Ever see Tenshi no Tamago? Putting it next to K-On is like trying to find out how tasty soap is! Things like that just aren't really fair to compare to each other.
Basically, yea.
I don't have one.
:x