What is a good adhesive for ceramic tiles?

You should not adhere tile on top of tile. It is much better to rip it up and install on a secure surface and make sure it is smooth level.

If you find yourself in a situation where you can not/should not remove the tile (e.g. if there is asbestos tile underneath or if there is a radiant heat systems under and you don't want to risk breaking/puncturing it), then the alternative is to pour 1/2" (or more) of self leveling cement and then install on top of that.