I wouldn't call them rivals becuase that implies something personal, but the guys that make me bear down and be alert - becuase you know you can't afford to make mistakes - are the most fun to fight:
Rondar (now my squaddie) becuase he's the best gunner I've ever seen.
A8Nisky because he can dominate a fight.
Dr7 becuase he thinks outside the box and is unpredictable.
Anyone who almost exclusively ups Tigers in base defense - becuase it annoys me.
There are lots of others who I just have'nt run into as much that are pretty darn good as well, but these guys stand out.