I love to play against horde armies, especially the ones that want to rush into close combat and kick your head in. It's a ton of fun to have a tidal wave of Gaunts or Boyz charging your lines while you frantically fire as many shots as possible into them so you can hope to counter-assault.
The armies I dislike playing against are the highly mobile shooting ones like the Eldar, Tau and certain Guard lists. Armies that don't actually want to fight (i.e., armies that refuse to engage you in close combat unless they have a massive advantage) and would prefer to spend the game running away piss me off.
When it comes to the armies I like actually controlling myself, I like really quick and deadly assault armies. Gunlines bore me - I like to get in my opponent's face and kick their *****, hence playing both Blood Angels and Daemons.