This is probably the most popular question here. Do a search on "best" and "favorite" front end and you will see several passionate arguments made for several.
Based on my observations here, the most popular three for Windows operating systems in no particular order are MameWah, AtomicFE, and GameEx. I believe these to be the most popular because they look nice and also seem to be the best supported.
Probably in this thread there will be passionate arguments for at least a couple of others. I have tried the three mentioned above as well as Ultrastyle which was my favorite because it looked neat and was incredibly easy to set up. It is no longer supported, though, as of MAME version .103.
You will also see a popular answer that is: you will have to try out a couple to determine which one best serves your purposes based on functionality and style.
I personally prefer Mame32 (watch the flames come out here) because it is the easiest to navigate a large number of games and make quick, on the fly, configuration changes to individual games without having to manually alter .cfg and .ini files.