As a history grad (although I didn't take many more than the required courses in American history):

Worst decade: 1900s. It's post reconstruction and pre-WW1. Resconstruction isn't terribly exciting but there's more than enough "to it" that you could study it for two months and not be bored out of your mind. I think the 1900s is just an incredibly anemic decade - between two eras and no one really cares about it compared to most of the others.

Best: the 1960s is an easy choice here (JFK, Vietnam, hippies, etc.) but it's almost everyones favorite decade and it's way overstudied. Let's go with the 1920s - you have a lot of economic prosperity, the end to the most retarded law in american history (prohibition) and reasonbly strong parallels between "corporate power" that you see in today's society. You can also theorize how the economic conditions in the 20s potentially lead to the problems in the 30s and 40s. Culturally it's at the beginning of cinema and big bands and there's some interesting stuff there. The 60s is so often studied that I think the 20s would be more interesting to study for two straight months hardcore. You can't go wrong with either though.

A couple asides: the 40s is not that interesting - we're talking about American history keep in mind. I think the 30s/40s would be more interesting from a global perspective; keep in mind the depression was a worldwide thing, and analyzing different approaches to it would keep you busy for a while. Also, the early 1800s isn't as boring as most people think, and I thought about being really different and suggesting one of those decades.

-dustyn