Tough one, I'm not even sure if I've heard all their albums, heard most of them, though. Blue Bell Knoll would probably be my favorite that I've heard, Heaven or Las Vegas is really good but I don't like it as much as I used to, I think Blue Bell Knoll has aged a little better.
The first half of Milk and Kisses is great, Tishbite is one of my favorite songs of theirs, but I think the second half shows why they broke up, unless it was just for personal reasons. Pretty boring stuff.
Pearly Dew Drops Drop is probably my favorite song.
I think Victorialand is their weirdest, most experimental album. Some really cool songs on there.