I use to sell cellphones for ATT Verizon TMobile. It really depends on where you live and how your going to use it. If you want all the bells and whistles than you will probably want the best data plan, which at the moment is 4G. Keep in mind you want a phone that is backwards compatible with 3G. Some areas only have 3G, in which case you'll be glad you have a 3G capable phone.
The two most common places you'll use your phone is your home and work. Ask your neighbors and co-workers what they have and how they would rate their service, and look at how many bars they have. Ask them if their signal drops as soon as they walk inside, or into the bathroom. Don't buy a phone/ plan just cause its a good value. Buy something that works! I use to have a T-mobile dealer plan, and an un-locked phone.. I love the phone but the over service was not that great (for my area).
In the Los Angeles area Verizon is the overall best. With that said for where i work and live I use boost mobile and i'm very happy with my service for the price im paying.