Any quality name brand polymer modified thinset should be fine. Some of my favorites: Custom Products, Mapei, C-Cure.

I noticed you mentioned tiling over a newer slab. I highly recommend you first put down a crack isolation membrane before you tile. Chances are good that the slab will develop a crack at some not so distant time and that crack will transfer through to the tile! There are many good ones out there. I recommend RedGard by Custom Building Products available at Home Depot and at many tile distributers.