T Mo has the best coverage for my local area. Its funny to see other people bring their phone inside of any building and hear them ***** and complain about how they have no bars. But T Mo is still strong inside of most buildings in my area. Pretty much all around town I have 5 bars.
But honestly there is no "best" Carrier. Different areas all have different strengths and weaknesses. One carrier might have the best coverage but the worst customer service *ahem* sprint, others may lack in coverage and have great service, TMO, Some may get every new phone there is on the market, AT&T, and still others may have great CMDA/EVDO coverage, vzw/altell.
Bottom line, pick the carrier that has the best coverage in your area and hope they have decent phones to pick from. No sense in having the latest and greatest phone but no bars of coverage.