I've been pretty happy with T-Mobile after switching from At&t. My girlfriend and I switched last summer when they had a really good 2 person deal ($80/month for 6 GBs of data each, which is much more than either of us ever use). She had slightly worse coverage on her iPhone, but my S7 Edge seemed to get better reception than on my Note 4 on At&t. We live in the Seattle area so coverage was pretty comparable.
My only complaint with them now is that they don't offer Motorola phones. I'm very tempted by the Moto Z Play's battery life.