The "Snaps" are now a very large franchise - even many banks use them for the procurement for each branch so the quality is improved dramatically.
It's too hard a question as it depends on print requirements.
All printers are unique.

I wouldn't call it the 'top' one, but Vista Print is undoubtedly one of the most popular. 

A google search will show that there are so many. Agree @Craig Gardner it will depend on what it is you are wanting, your budget and timeframe.  Top ones would be Minute Man Press, Snapfish, CMYK Online, The Print Company, Vistaprint to name a few.