Everyone has their favored army to play - whether it was your first army, or a newer one you've come to love. But what's your favorite army to play AGAINST? The one that gives you the games that are the most fun, or the biggest challenge?
For my Chaos Space Marines - Loyalist Space Marines and Sisters of Battle. I know, I know, MEQ vs MEQ blahblahblah, but those are the most thematic for me. Against Space Marines, because the Long War still endures, and every fight against the Loyalist dogs of the Emperor is another fight closer to ending it. Sisters of Battle, because I just KNOW that Chaos Space Marines see them as pretenders to a legacy they do not deserve - trying to be something they can never be, etc etc. I think that's how the Chaos Space Marines view them - and besides, from a gameplay perspective, Sisters of Battle are awesomely fun to play against when I bring my Thousand Sons.
For my Chaos Daemons...I don't know, really. Every game is different with them, and I haven't gotten to play them as much as I like.
For my Necron? Tyranid. Definitely Tyranid. More specifically, horde lists of Tyranid. That bit of fluff about the Tyranid purposefully avoiding random worlds in the path of their hive fleets, even if they were teeming with life, just made me laugh. Even Tyranid know to stay away from the Necron, and besides, against Horde lists, nothing says "go away" like some Gauss Flux Arcs.