Slitherine

I still favour plasticard - 60 thou for 15 mm and 80 thou for 25mm. It is getting harder to find as the little model stores are closing down.

It cuts easily (score with a sharp scalpel and bend), doesn't bend (unless you use 40 thou which is too thin) and slides well across the table when you are moving blocks of figures.

I coat with tetrion to cover figure bases and provide some surface variation then paint grass green or sand, wash with earth brown or dark green, dry brush with a lighter shade of green or brown and then paint the edge of the base in black.

Easy to do and looks pretty good (I favour the sand/brown variant but it doesn't look right for north european troops like my Romano-Britons)

Litko stuff is great
They have three thickness like was posted earlier BUT the plywood edge is exposed - which drives me crazy (short trip)
check out Gail Force Nine
They have bases that you can get in bulk
They are cheaper and they are made from masonite - so they don't have to be finished on the edges
They are more like 3.5mm or 4mm thick
I love them
I really like having my 15mm figs on bases that are thicker SO it is more like a chess piece - so you are NOT always picking them up by the head and damagin stuff