I use different labs for different things and redundancy (kind of not putting all of the eggs into one basket):
APS Pro Lab (in NY) - great quality and costumer service!
Miller's (I don't use MPIX and MPIX Pro - For one, I find that MPIX quality is far inferior to its pro division; the price difference b/n MPIX Pro vs Miller isn't significant enough while the quality and costumer care of Miller's is far superior to the other two - just my experience).
Black River Imaging - They are ok (price, costumer service). Quality: I've worked with them for few years and in the past month noticed print quality issues on large prints (above 11x14). At first I thought it was the camera so had the same prints made somewhere else but it wasn't my gear. The problem isn't something obvious unless you know what to look for and/or have bases to compare to. I still use them though.
H&H - used them for a little bit.
WHCC - I used them a bit, I like their quality but on some things price was more then I was willing to pay.