I have used Verizon, Sprint and now T-Mobile and I can say that Verizon works the best in all boro's but Manhattan. T-Mobile has LTE in all places in NYC and is not spotty. Sprint is spotty because they haven't finished upgrading.
My gf who uses AT&T has great LTE service but in Manhattan, there is issues of it being slow due to capacity. Depends on where you will use it the most, it's either Verizon, AT&T and T-Mobile.
As AutoUnion39 says all the time, T-Mobile is all 2G/Edge outside of big cities. It's not all edge/2G but you will notice it more on T-Mobile while traveling compared to AT&T/Verizon.