When it comes to flattening wood slabs using a router, a straight bit or a leveling (surfacing) bit is commonly used. Straight bits with a large diameter and flat bottom are ideal for initial leveling and removing excess material. On the other hand, leveling bits are specifically designed for slab flattening, featuring a wide cutting diameter and multiple cutting edges. These bits are effective in achieving smooth and flat surfaces. It's essential to choose a router bit suitable for the size of your wood slab and consider the desired outcome of your flattening process.