I know people have reminded others, but the question was
America's greatest active band.
If it was 2 albums ago, I would have tossed out a second-place vote for Kings of Leon. In my opinion, 3 albums ago, they went from raw bad-ass, to relatively soft. . .which was confirmed with their last 2 releases. Their first 2 full releases were simply awesome. To an extent, I would describe their first 2 albums as a somewhat modern take on the best-of-the best southern rock music from the late '60s and early to mid '70s. At the time, I thought their 3rd release to be somewhat of an anomaly. . .a bit of a homage to the desires of Lord Record Company. From there on, however, it seems to me they
really pushed hard to break the US market (being quite popular in the UK, but generally unknown in the US). With that, they became quite 'poppy.'
Even ignoring the latest releases from KOL, my #1 vote would have to go to Wilco. Their latest release just continues the awesomeness. I am a big fan of all their albums, which of course includes stylistic changes over time (more so, in terms of broad changes from their earliest stuff to Yankee Hotel Foxtrot). At the earliest, Wilco could simply be described as an alt-country band.
For me, after the break-up of Uncle Tupelo, Son Volt's
Trace blew Wilco's first effort out of the water. From there on, however, Wilco grew. After several years, I would even say they
evolved. For me, the only comparison, in terms of such a musical evolution, would be a comparison to The Beatles. The only other band to successfully make such an evolution (of course, in my opinion), has been Radiohead. However, I think Wilco's evolution has been more musical. As a result, Wilco went from #3 on my list of favorite bands, to #2, switching spots with Radiohead.
In a way, Wilco's latest release tied
everything it released beforehand all together. Their latest,
The Whole Love, brings in subtle experimental elements (a la
Yankee Hotel Foxtrot and to a lesser extent,
A Ghost is Born), with their generally evolving alt-country roots.
Not to turn this into a Wilco review, but I'll say Tweedy (and as far as considering general band dynamics, Stirrat, who is has also been around since day one) has/have definitely benefited from hist/their current line-up,
especially with Nels Cline.
I am very much looking forward to seeing them once again, for the 5th time, on December 3 in KC!

Due to a weeknight concert, I couldn't make it in to St. Louis (STL) several weeks ago for a show there. For those that don't know, Wilco is based out of Chicago, but Tweedy (and Farrar, from Uncle Tupelo, now Son Volt) are from a suburb of STL.
Paul