What criteria are you using besides
1. Gossip in the halls where you work?
2. The halo effect for Ivy League names?
3. Your own personal book reading preferences?
I think this is an interesting thread, but it also reveals how nebulous, and somewhat predictable, such rankings are.
I would also like posters to indicate how many university press books they've actually bought in the last year. I think I've bought 5, maybe.
The Fiona
I just did a quick visual inventory of the bookshelves where I have most of the leading research books for the period that I study, which is early modern English lit, and books that I use for my two current research projects. So not hard empirical research, but a fair reflection of which presses are publishing the most influential books at least in my field (Chicago, Stanford, Cornell, Cambridge). But there is quite a bit of overlap for those top presses in other MLA fields, too. It's true that I don't buy a lot of books -- mostly ones for teaching and my research, and almost all of those are from academic presses.