As with most things in life, you get what you pay for. The most expensive service I've tried costs over three times the cheapest, but it's well worth the extra cost in my mind, especially as even high-quality prints will generally cost less than the frame you will put them in.
My best advice would be to avoid using any service you've heard of (especially if they begin with a B or a J...)
The one exception to this rule is Snappy Snaps, which have produced very nice images for me and are very reasonably priced.
Photobox do nice photo books and calendars with a very easy to use interface.
But if you want the best, I would use either:
Peak Imaging. Choose Satin Matt/Gloss Prints (http://www.express.peak-imaging.com/) for a classic look or go for Fujiflex High Gloss prints (http://www.fujiflex.peak-imaging.com/) if you want a luxury, more richly coloured image. Colour: choose Lab Optimisation
LoxleyColour (https://loxleycolour.com/roesprints.aspx).Make sure you request colour correction.
A quick tip:
- Watch out for clipping of your image if you don't choose a size which has dimensions in the ratio of 3:2 (many sizes are squarer than the 3:2 of a photo, in particular 10x8, and these will lose the top and/or bottom of your image)