The one I'm most proud of is the Chaos Coalition, a group of superpowered mercenaries. I designed each member to exploit the weaknesses of two PCs, but be vulnerable to one other PC. The team consisted of:
Omega- A gadgeteer with stasis grenades to stop our teleporting martial artist;
Slag- A giant lava creature to counteract our ice generating powered armor PC;
Pulse- A being of electromagnetic energy designed to stop the hero who controlled Kinetic Energy;
Poltergeist- An Invisible, desolid mentalist designed to twart the team's invisible, desolid mentalist;
Golden Tiger- A martial artist with missile deflection/reflection to use against our archer;
Gary Gillman- Guy Gardner clone created to fight our Kyle Rayner clone;
At the end of the story arc, both teams faced off at the MGM grand, and the whole battle was fought on Pay-Per-View...