In my un-informed opinion, a good sealer is one that beads water on the grout.

I certainly am no expert.

I used one recommended to me by the Home Depot tile guy, it was an aerosol product that had a huge amount of fumes, I did 3 applications of it trying to keep it mostly on the grout lines, so far it seems to work good for our bathroom floor tile as far as keeping the water out of the grout.