Radius cutter, however 8mm may be a bit big for anything but a reasonable size machine with a relatively slow spindle.
I've just checked the first cutter I found (arc euro trade), and an 8mm radius cutter is 24mm OD, with 8mm centre, and a 16mm shank, so a failry substantial machine would be needed.

Removing the bulk of material using a normal endmill and several passes would mean minimal cutting for a final finishing cutting pass.

Other option would depend on how good a radius you want. You could probably get a reasonably OK finish using a ball end mill and multiple passes.