I'd either guess the MacBook (smaller screen = less battery drain) or 17" MacBook Pro (biggest battery).
Of course, that's not really helpful, since I just named 2/3rds of Apple's laptop lineup
from my experience, with a CoreDuo Macbook, 17" Powerbook, and 15" MBP:
Macbook can get up to 4 to 4.5 hours with select usage (dim all the way down, not running a DVD or doing anything terribly CPU draining)
17" PB would get about 3.5 to 4 hours easily
15" MBP can get at least 3 hours easily.
So short story, Macbook would get longest battery life. It also has the smallest screen and is slower, too. You'd probably not notice much of a difference between that and a 17" MBP.