Just among the 4 major carriers. So excluding prepaid, MVNO etc...
My kids are both in college and they want to take their lines away and go off and get their own service so they can upgrade from their handmedown iPhone 4's and get shiny new iPhone 5's for $99 plus $40 a month. PLEASE. I explained to them that 4 lines of iPhones costs upwards of $250 on At&t or Verizon and a single line would cost even more (per line).
The 4 companies you mention are all tricksters. I've used all 4 of them over the years, indirectly if not directly. The following are some of my impressions:
Tmobile (they were called Voicestream at the time):
Very spotty coverage (I've heard it's better now)
Lowest price
Best international roaming these days (I've heard)
Ability to make calls on wifi (I've heard)
Nearly nonexistent LTE
Allows "payment plan purchase" of phone without "subsidy" in plan price.
Sprint (I had 2 Virgin mobile phones for my kids at one time)
Awful coverage, frequent dropped calls
Every change to account incurs a new contract (this is what Sprint reps repeatedly told me when I called to ask about a new account)
Aggressive pricing but arguably not as low as Tmobile.
Nearly nonexistent LTE
At&t (I have been with them several times over the years)
Decent coverage
Recent "no contract" discount on smartphones of $15 a line. I have YET to see this actually show up on my bill (but customer service has issued credits until they get billing straightened out).
Decent LTE
Fast 3G speeds
Clueless customer service, among the worst in any industry
Able to change plans without extending contract (from recent experience)
Verizon (I went from Verizon to At&t)
Absolute best coverage
Best LTE
Absolute highest pricing
Best customer service of any phone company
Able to change plans without extending contract (from experience)
Rather junky feature phones and replacing a broken one too many times leads to new contract, hence our move to At&t and all iPhones.
So how do I vote in your poll? I don't.
Buying cell phone service is a lot more difficult than interstellar faster than light travel.

At this point, I would say it's best to save up and buy the unlocked iPhone you want then pick a PAYG "carrier" to use it with such as straight talk. The $40 "fee" for having a smartphone on an At&t account is reduced to $25 if you bring your own phone. So if you pay $450 for an iPhone, then bring it yourself, you save $360 over 2 years. If you had paid $99 for that phone up front your out of pocket would be $460 so if you can find the phone of your dreams for $450 or less, it makes sense to tell At&t to stuff the contract and buy it yourself. Sadly the cheapest new iPhone costs more than that. If you're willing to deal with the snake pit that is ebay, checking completed listings I find iPhone 5's sold in the $450 range and I spotted a handful of iPhone 5c's that sold in the $350 range. So for what you would have paid At&t in extra fees for having a subsidized phone for 2 years, you could own the phone outright up front and get a little something for it at the end (since it's unlocked). Iphone 4's are going for around $100 on ebay these days.