The basic answer is something sturdy. Also something that sits firmly on the floor. You can't afford to have wobbles or vibration, but many small tables will do the trick. I have a very small darkroom, and I use a small desk to hold the enlarger. It's handy because I can use the drawers for lenses and paper and stuff. I attached a little shelf to hold the timer and the color analyzer, and I braced up the back overhang of the enlarger copyboard. The desk provides a good working height, which is important. Many find it better (more sturdy) to remove the enlarger from the copyboard and attach it to the wall. The same requirements for firmness still apply, though. You can't have the image move on the paper, which means the paper must stay still. There are some commercial tables made, and they have some advantages.