I have tried about a dozen American Oktoberfest beers this year, and was impressed by only two of them:
Capital Brewing, which comes about as close as you can get to an authentic German version, and
Leinies -- thin but reasonably close.
The Sam Adams also tasted good, but also seemed faked, and way way WAY too much caramel malt, so much that it was super thick and chewy.
There really is nothing else like an authentic German Oktoberfest. All the German examples are awesome. My favorites would be Hofbrau and Hacker-Pschorr. I haven't had the Ayinger in a number of years, need to get my hands on some for comparison.