It really depends on where you live and what sort of usage you need.
Verizon is the only service in my area worth anything. They are the only one that gets reception inside my house in my findings. My sister in law has AT&T, and she couldn't place a call in my house she had to walk to the end of the driveway actually lol. Sprint, can get reception but not well inside my house, it is available on my back deck and quite well. The two places I spend most of my time, work and home, Verizon is the winner at both. It Verizon also currently has the fastest 4G network and soon to be if not already, the biggest 4G network as well.
As with most things in life, you have to pay a premium if you want the "best". If you live in an urban area you can probably get by with one of the smaller companies like Virgin, MetroPCS, Cricket. But if you go to the suburbs, you may not have such good luck.
To answer your question though, if I were to leave Verizon I guess I would go to Sprint as I really don't have a better option. Sprint's lineup of devices is rather nice. I have heard decent things about their customer service, they have decent pricing too.