I generally play on auto-match, so I get a wide variety of opponents, and I haven't played any one of them enough to have a large enough sample size to be able to label one a 'nemesis.'
(btw, I'm always up for an invite game with any player of any level. If you're ever on and see paddyodoors feel free to invite me.
Also, I'm trying to improve my game -- so if you're a top-player or even a higher-rank, I'd love to play against you for the sake of learning.
And if you're lower level, I promise I won't bite, I just enjoy playing and I'm more than willing to share whatever tidbits I've picked up along the way.)
One player makes me feel like he is my nemesis is fellowmartian (0-5-1)... he is the only player who I have never beaten after at least 5 games. But like I said, small sample size, so we're not in nemesis territory yet.