Generally, imo:
If you want coverage, go with AT&T and Verizon. They are generally reliable in almost everywhere. Except Verizon has a slight edge over AT&T.
If you want speed, AT&T offers faster speeds in areas where it has coverage as comparable as Verizon. Although I'd say, the advantage over AT&T here isn't really that significant. With LTE especially on a smartphone, a 30mbps download speed over a 20 mbps download speed isn't really significant in reality.
If you have T-Mobile LTE in your area, it is blazing fast. Considering less people uses T-Mobile LTE in its LTE markets . The same can be applied to Sprint as well, but I found that Sprint's LTE can only reach an average maximum of about 20mbps, while on T-Mobile/AT&T/Verizon I can get past 30 or even 40. Of course, this is just my experience.
If you're budget conscious, T-Mobile or Sprint is fine. Now it all comes down to coverage. In terms of general coverage, both carriers are fine. (NOTE: You DO get coverage with all major carriers, its just the speed) . But when it comes down to speed, Sprint is out of its league. Go with T-Mobile if you want a generally reliable coverage and speed. But don't expect excellent coverage like Verizon or AT&T.
Do not go with Verizon and AT&T if you're budget conscious. They're the most expensive carriers in the market right now. This is of course 'compensated' by their generally excellent coverage and speeds.
Unlimited Data? Go with Sprint, although I am not sure you can tolerate its sluggish speeds in most of its markets without LTE.
Best Value (Not too expensive, average/good coverage and speed etc) currently to me is T-Mobile.