i have had terrible luck with snapfish, The quality and color were off, and i hate getting them in the mail.
I love Walgreens, They really have true to life color, and i like the ease of using their website to edit a bit, add borders, change to black and white, that sort of thing. I love the option to pick up instore because it is so convenient an they don't get wrinkled and squashed in the mail box.
I wait for sales, one should be coming up around fathers day. I can get 10 cent prints, they usually have sales where you can buy 75 at 10 cents, or I usually wait and do 50 at 10 cents since i do them often and sometimes don't need 75.
so My vote is definately Walgreens, over snapfish ( or shutter fly for that matter) that website is hard to use and the quailty is poor, haven't tried costco. but i imagine they would be a good choice too.