Unless you're doing art prints, or can spot the difference between black and black, then most of them of them are much of a muchness.
We use photobox, as they had a free print offer once, and have a quick turnaround.
However, for everyone who says use them, they'll be someone else saying don't. Best bet is to use them all for any free offer they have going, or a small order, and then choose which one you want for the rest.