For anything but fine art print, I love mpix. Easy to use site, mostly very good prices, wide range of products, lightjet output on great paper, EXCELLENT turn around time and shipping costs.
WHCC also gave me great results, but the prices are a little more, and the site isn't as easy to use.
EZprints has given me hit or miss quality in the past. I used to use them for larger prints because of the price - but sometimes there would be obvious horizontal lines - like no one actually checked it before shipping.
I have never ever had a quality issue from mpix, and every order has been 100% perfect. They are so far the only vendor I can say that about.