They all suck.
That said, ATT is the best for international.
Beware of data usage overseas and Japan has a unique system that will not work with any American phones. If you travel there, rent a phone for the duration of your visit.
http://www.jcrcorp.com/ are good.
FWIW, I’m an IT manager for a VC that travels extensively overseas.
Worth saying again, watch your data usage outside of the US.