I've been on Verizon, Sprint, and T-Mobile prepaid in the past, on Solavei now and haven't regretted my choice at all.
If you are in an area with decent T-Mobile coverage I'd highly recommend it for a few reasons over the competition.
1.
Support for conditional call forwarding (unlike most pre-paids you can actually use fully integrated Google Voice or Google Voice Lite)
2. Clearly defined data limit that's higher than most comparatively priced plans (4gigs before any throttling per TOS, I've yet to experience throttling though)
3. Current promotion offers free SIM for those bringing their own device and first month of service free (second month of service free if you have 3 friends sign up in addition to the 20 dollars a month you'll still get after that effectively making your bill only $30 a month for Unlimited Voice/Text/Data)
4. 'Social Membership' allows you offset your cellphone bill either in part or entirely. If you enjoy the service you can tell others and earn money. If you don't then it's a prepaid service with no contract so you can try another MVNO.
Side note: 'Social Membership' is optional.. if you don't want to tell other people about the service you don't have to, you can just enjoy the service at the already decent price.
Solavei (referral link)
If you don't live in an area with decent t-mobile coverage then Straight Talk with an AT&T SIM has worked mostly well for some of my friends (although at least on the Columbus Ohio area they seem to get throttled at limits that seem based more on which part of town they live on than strictly data usedOne friend hit 'cap' at 500mb while another was at 2.5gb and still not throttled)