Kazoo would pick snipers
Mini sentries, but only because they're nearly impossible to take down as a scout, unless distracted by something else, and they build so incredibly fast that its hard to target engines and deal with them at the same time.
But from an engies perspective, they're very helpful
I feel that the dead ringer perhaps deserves a nerf, but I don't think it needs to be removed from the game. The idea is sound, but for instance, the fact that you are all but invincible while cloaked is not. If the spy were to take normal damage while cloaked, this would make much more sense. I suspect, though, that the reason this is so if a consequence of lazy coding more than anything, and that it will likely never be changed.