Oh, I love the Problem of Potions!!! It's one of my favourites--imho it does a really great job of everybody, including people you don't often see done well together (Draco & Ron, say), or done well at all (Dumbledore). And yeah, Draco's arc is well-done and believable :) And Snape! I love how he's just as much of an asshole as he is in canon, but you still get to see him develop. And their take on Voldemort is amazing. And Harry, of course, who is a really compelling character, and who holds the whole thing together <3

...sorry about that, apparently I'm feeling like gushing a bit :P