I've seen some bad ones online, and some not so bad ones... Batman vs Predator makes sense, but then I see awful shit like X-Men and Star Trek :/ and then there's the more obscure and hard to classify ones, like Jesus kicking the shit out of the Greek Gods for heckling him at his crucifixion.
I'm bloody sick of crossovers that make zero sense and don't add up in their collective settings. So post your own here.
I'm sure I'll have some later, but right now, all I have is Assassins Creed: Dance of Killers. While one of the Brotherhood's Assassins moves in on his target, an unseen foe wipes out his quarry. He finds when he's on a contract, he is shadowed by a Predator, and when the young hunter grows bored with the targets, he turns his sights on the young Assassin.