Tiling an uneven cement floor.

What substance is BEST to use on a VERY uneven and old cement floor to bring it level for future ceramic tiling? Would it need a subfloor on top after?

If cost is not an object or its a relatively small area you can pour self leveling cement on the floor. That answers you direct question. What condition is the floor in? Does it have any cracks in it, it is painted or sealed with something? Is it constantly damp or wet? These questions need to be answered before you do any work as other prep work may also need to be done first

How large an area?, if you have the height, mud job.

The floor has no cracks but is painted. The area is small but is near a drain so it might get damp.

So- - - - - you don't want us to know how big the area is? Is there a slope to the drain and if so is the slope to remain?

And have to lose the paint.