These nominations are always skewed by who you have been fighting the most, but fact remains deuce22 and Rastafla are the only ones I can think of that stand out when it comes to sportsmanship. You can see the non-oral proof in their battle result posts against me. There are other pleasant opponents, but these two stand out.
As a third nominee, I'd pick Kuroaitou. He hasn't had the chance to play a lot, but he's always super-nice.