I've always had my favs, and these are them....
1. Iron Warriors (although I've never built an army of them) For the siege aspect.
2. Blood Angels for the assault aspect of them.
3. Salamanders cause fire is good.
4. Emperor's Children... who wouldn't want an army that would make KISS proud? I always thought an army with KISS as a theme would be good for ECs.
5. Rainbow Warriors, oh the fun you can have with a theme army there...