Ayinger has a decent Oktoberfest (Marzen) beer
This year's is too sweet, but Sierra Nevada's Oktoberfest beer changes every year in collaboration with a new brewery, and some have been pretty good
Trader Joe's spiced ale (winter) is pretty good, it is brewed by Unibroue but only sold under the Trader Joe's label
Leinenkugel's has two that I like, though I think there is some rose tinted glasses effect from when I drank them.
Their vanilla winter porter I remember liking, but when they first rolled it out, the first few batches had a very bad lactobacteria infection that I don't know how it got passed QA. As a result it was often heavily discounted for a long time afterwards while I was at university. So it was one of the cheapest beers I could get and drank at parties.
Also Canoe Paddler, their summer Kölsch I remember liking, but I have only drank it while vacationing on a lake in the summer. So that might have influenced my positive view of it.