In article 2017021620135464924-savageduck1@REMOVESPAMmecom,
Savageduck wrote:

I can only tell you that while Verizon has good coverage with at least
two towers where I live, T-Mobile is mediocre and shows no towers, and
both AT&T and Sprint are poor to non-existant, also showing no local
towers. This leads me to believe that T-Mobile, At&T, and Sprint are
actually roaming in my location, using the Verizon towers.

sprint probably is, but att/tmo aren't.

AT&T coverage out here at Lake Nacimiento, West of Paso Robles, is
mostly bad to non-existant. That was my reason to not having an iPhone
until Verizon came onboard, and I seriously doubt that they are making
any effort to solidly establish their presence here in any competitive
way.
T-Mobile does have much better rural coverage than AT&T, but nowhere as
good as I get with Verizon.
As to whether or not any of them are using Verizon towers for roaming,
that is only conjecture on my part.

it's not conjecture on my part.

sprint/verizon have roaming agreements, as does att/t-mobile.

historically, sprint/verizon used cdma while att/tmo were gsm/hspa. the
two air interfaces are not compatible, so they *can't* roam on each
other's network.

now that all four carriers support lte, it's technically possible,
except that each carrier uses their own set of lte bands and not all
phone models have all of them (i can think of only 3-4 that do), so
it's not a realistic option. if they did offer it, most people would
not benefit.

roaming is usually just for voice/text, not data, however, it depends
on the specific plan, and there may be additional fees in some cases.

tl;dr - coverage varies. choose the carrier who has coverage in the
areas in which you travel and at a fair price. do not count on roaming.
there is no single 'best' for everyone.