Uhhhh,,,, looks at collection.
I'll base this on standalone volume merit, rather than a favourite series...
I'd say Scott Pilgrim Gets It Together Vol 4, for some reason that's my favourite of the series.
As a completely standalone book... actually Lost at Sea is pretty nice, I guess I really am a fan of his.
I do like V for Vendetta as well.
Special mention to Making Comics by Scott McCloud. Only reason it isn't there is because I consider it more of an instructional book that just happens to be a graphic novel.