With Club teams in master league it is Man U. They picked up Ballack in midfield and even though they dont contend for the title they play me very tight. Although they just lost Ferdinand to Juve on bosman, lol. It used to be Madrid, but I just beat them 6-0 on agg. in the CL semi. 4-0 in the away second leg. I think that squashes that rivalry.
National teams has to be Czech Republic. I always seem to have trouble finishing my chances and with the counterattack or Poborsky/Rosicky/Nedved through midfield they are tough to contain.
Sidenote, on old WE6:I, it was Germany. Deisler would score at least one freekick on me and Neuville would bag the other. Then they would just shut me down.