Oooh, interesting question. We're getting pretty close to the half-way point for those of us who count '10, so while we have whole lot of time left, this isn't a bad time to start thinking about it!
A couple of my all-time favorites just miss the cut-off date. That would have made it easy. As is, I'm left torn between Neil Young -- Americana, Tom Petty -- Mojo, and Katy Perry -- Teenage Dream.
I should probably be giving it to
Teenage Dream on the basis of it by far the most relevant and influential entry, one from an artist in her prime. It's about as flawless as any pop album I've ever heard and it really set the bar for dance pop. That being said, its modern immediacy is a double-edged sword. Katy helped pioneer the wave of EDM-based pop but Teenage Dream was just the start, and now I find myself more interested in newer albums that take that angle to a higher level. I think years from now I'll probably rate Teenage Dream where it deserves, but living in the moment it's hard for me to not prefer the newer works, kind of like comparing early Beatles to Sgt. Pepper.
Tom Petty's
Mojo is the album I listen to most out of the three. It's got some truly spectacular tracks and really builds on what Mudcrutch began. But ultimately it just doesn't come off as a "best of the decade" album to me, because it's sandwiched in between two other records, one better (Mudcrutch), and one just as good (Hypnotic Eye). Mudcrutch was the real innovator where Petty started getting into this sound and that's the one that would deserve the highest accolades -- but it's not from the 10s.
So I'm left with
Americana as my choice. No big surprise whenever a Neil Young album ends at the top for me. This record really hits on what makes Neil and Crazy Horse great, just raw power and no foolin'. Their first record in six years and boy is it good to have them back! The material on the album strikes a cosmic chord with me. It's one of those very rare entries where a legacy act can re-inhabit the sound of their youth.