That's a loaded question. Best? Probably machined cast iron, or maybe granite. Too expensive for me, though. 2nd best, probably solid phenolic, still pretty expensive. Do you have a budget? High-pressure laminate (like Formica) on MDF is a good choice, but is not impervious to moisture issues. Low-pressure laminate (melamine) would be the low-budget choice, but not much improvement, probably, from what you've got now.
I've got a melamine router table, and a HPL table. Both work fine for me. Thickness really only matters in terms of reducing/eliminating sag---structure under the table will influence this (better, flatter sub-structure = less need for thick top and vice versa).
I used 1/2 of a solid core door, turns out "solid core" means it's particle board. But that was about 12 years ago and it's still flat. I think the more common material is MDF, 2 pieces 3/4" thick laminated and covered both sides with HPL (I did that to mine). But thwe real key to keeping it flat (IMHO) is to have a grid work built into the cabinet supporting the top, my table is a version of what Norm built and it has done quite well.