I can't say I'm utterly opposed to crossovers, given my first fanfic was one. (Granted one I'd copied off someone else, but still.) Done well, I think it can be really interesting, but then I'm all about winding characters up and seeing where they go - it's a mix of the familiar and the unfamiliar that often works for me - whether that unfamiliar is a situation that stresses people/relationships, an AU or a crossover.

I haven't written that many crossover fics, but if I had to just choose one fandom to crossover with anything and everything, it'd have to be Amber. Partly because I'm way into that fandom from the roleplaying side of things - and the roleplaying game actively encourages and incentivises you to write fanfic for your game - and also because it's intensely easy to cross Amber with anything given that Amber contains an infinite multiplicity of worlds. I've only *actually* written two Amber crossover fics though.