AT&T for me. Switched to AT&T about 9 months ago after 10+ years with VZW. First priority - terrible signal from VZW in my home, good signal with AT&T. For travel in Mid-Atlantic, NE, SE and upper midwest, I have had very good signal. Also good for trip to Las Vegas, UT and AZ.
I tried to "upgrade" my old grandfathered plan with VZW, but this was not possible unless I bought new device w/ contract or paid a lot more. An irritant was that my old plan did not include text messages. I needed (and now use extensively) texts to communicate with adult kids and grandchild, plus I needed text for 2-factor authentication and security notifications for credit card and bank accounts.
AT&T (in the last year or so) has been quick to make competitive offers. I now have 4 devices (2 iPhones + 2 iPads) sharing a 10-GB data plan (now 20 GB) with NO CONTRACT, unlimited talk & text. I have already taken advantage of 2 competitive offers, $400 in service credits to activate the 4 devices and a "free" upgrade to 20 GB of data per month. VZW? no way, just a bad memory.
I recently bought a new iPhone 6 that was technically placed on the Next plan, but I paid it off after the first billing cycle and went through the relatively easy automated procedure to unlock the phone.
I'm pretty satisfied with ATT - better signal at my home (compared to VZW), much less cost, more data, no contract, GSM network, easy to unlock phone. VZW takes extensive measures to lock in customers through various means - incompatible network, contracts, rate plan structure, not allowing outside-purchased devices on network, etc.
ATT - OK. Verizon - never again. Learned my lesson, won't forget it. Would consider TMobile if their signal was better outside large cities.