I can't actually remember the first time I stumbled upon lupin_snape (even though it would have been less than a year ago), but I do remember being struck by how helpful, witty, and supportive the members were. Which was a pleasant surprise considering they were all so clever and damn talented! I fell in love with this wank-free comm, and they haven't been able to get shot of me since!
I ship Snape/Lupin simply because they are made for each other. The fact that they also happen to be the two most interesting characters in the HPverse is a lucky bonus! ;)
Most memorable fic? That's a bloody difficult question because 1. the standard is so damn high here, and 2. my memory ain't what it was! I'm really enjoying werewolf_lib's The High Cost of Living, which must be memorable for me, as I'm not usually a fan of episodic fics (too impatient!)
Most memorable art? Again, why do you ask such difficult questions? In amongst all the other gorgeous pictures, the one that has come bouncing into my head is karasu_hime's picture of our beautiful boys with Sirius in fairy wings as their matchmaking godfather.