I'm about to take the ACE Certified Personal Trainer exam, March 16th. It is one of the big ones, like the others mentioned, and many gyms accept it. The material seems pretty well rounded, because it is assumed you will be working with the average person, and covers some special needs, like people with diseases that affect there physical ability, or pregnant woman, and young and old people.
I am planning on getting certified through a NASM, ACSM, and ISSA in the future. The more certs you get, the more opportunity at many gyms, and several I've talked to pay more according to how many certs you have.
I can't say one is better than another, partially because I don't have them all, but I've talked to a friend that is certified by 4 of the ones mentioned, and he says they all have their string and weak points, so combining them makes you more well rounded.