UHMW polyethylene sheets. It's the next best thing in providing a durable, smooth surface right after PTFE, but costs only about 1/4 as much. You can usually get it in white (natural color), green, and black.
I like green which gives the table a very nice appearance (I'd abstain from using black in fear of it possibly staining wood, although UHMW-PE is quite durable).
You can probably just use MDF, too, and it "will work" -- but I like the fact that you have very low friction sliding your wood over the PE. Low friction is more important than "super hard" if you ask me (you're not going to use the router table as anvil, are you!).
Accidentally put a piece of wood onto your nice shiny table with wood glue running out? Which you shouldn't have done, but which happened anyway? Well guess what, unlike MDF, the PE couldn't care less. That's true for most chemicals, by the way. Finding a chemical that will piss off a PE surface is really hard.
Be sure to pay attention to what exactly you buy, often when you enter UHMW-PE in a search box, you are offered HD-PE. Those are not the same, HD-PE is much inferior, pretty soft, and it will easily scratch (and rub off, which is unpleasing!).
You can cut PE easily (with a sharp knife if nothing else, but a saw will do just fine, of course), and it drills nicely and will play along well with the router in case you want to inlay a C-rail or do some other fancy stuff -- turn down speed and preferrably use a bit with one edge.
Unluckily, thanks to its special properties, UHMW-PE is also a real bitch to glue. Not only is it almost perfectly smooth, but the material also has a very low energy surface. I'm not a chemistry professor, so I can't explain what it means in a scientific sense. In practical terms it means that no glue whatsoever will stick to it unless you rub it with a special activator first (some people say sulfuric or phosphoric acid and heating will do as activator, but I haven't tried that).
Even after activation, only few glues will really hold reliably, cyanocrylate and polyolefin glues being the primary choices. I recommend adding a few small counter-sunk screws to give the sheet a bit of permanent push-down, just to be 110% sure.
There's people who recommend phenolic resin coated plywood as surface for router tables. I've not tried that, but it might be an alternative. Harder than PE, but less smooth (and less cool looking).
