I personally like AT&T's international plan the best, and is part of the reason why I stick with them.
$10/day for unlimited internationally, capped at 10 charges per billing cycle, after that it's free. For secondary lines on the same day, it's $5/day/line.
T-Mobile's 15gb plan for $50 isn't bad, but I personally don't want to have to worry about data consumption. And there's no way I want the slow speeds of their free data after 5gb. Still, 20gb total for $50 is more than enough for most, so pretty good deal.
Verizon, as far as I can tell, has no limit to the number of charges in a billing cycle for their day pass, and their $100 package is limited. Verizon also places limits on the countries you can call (only where you are and back to the US), while AT&T includes every country on their list for free.