Like them all for different reasons. Not sure I can truly pick a favorite. As Cosmic pointed out, if Boston had made another album or 2 like their debut, they could crush the competition. And i say that as someone who likes Don't Look Back as well as Third Stage... but neither of those matches up to the incredible monster that their debut album was.
It's really a tossup for me between Boston, Foreigner and Journey. Toto is the only one (for me) that is a solid #4.