I set my auto-match preference to +/-10 and have stayed within the same few levels for a few months now, so I feel like I have a lot of nemeses in the sense that there are a bunch of people I've played lots of games with. Everybody is friendly and fun to play with, and I could give plenty of different names here.
Most recently, mcmc has been an awesome frequent opponent, probably the nicest guy I've ever met on the internet and the player I'm most excited to see propose a match. He's risen up in level quickly since I first played him and is always an interesting opponent. We have very different play styles, which leads to some really cool discussion, and ultimately we run very even, trading wins often and staying close to an overall tie.
Eli_K is the guy I've played the most games with by far. I'm 54-41-2 there, but I feel like overall we're very evenly matched, and that eventually that gap will close. Mean Mr Mustard and I have played 50 games together and stand at 26-24, so I always know I'm in for a competitive match when I see him.
panda, ddubois and Tahtweasel stand out for me as players who I really have to be on top of my game against or I'm bound to get slaughtered by. Those are the Wandering Winder/Marin style matchups for me where I get overwhelmed and frequently schooled by big engines that end explosively or strategies that are simply much better than mine. Throw RomaNorgy in there too, and especially dghunter79, who is essentially my kryptonite. Winning against any of them is especially satisfying.