ONE?!?! that's a really hard question! It'd have to be a band that has a decent amount of songs and a good variety of them so they'll fit different moods and situations.
and one I could never grow tired of.

Dashboard Confessional comes to mind but I can't say it's my definite answer.
maybe The Smiths. or Stars.
I also love The Killers but I think I would possibly get tired of them.