I think it would not be one frame. Just as you don't have only one model of tank, and one model of ships, you would have a array of different robots optimized for a specific role/terrain.
- Need to deal with civilians, and you don't want to make them afraid? You don't need a lot of firepower, but have something they can relate to, so a humanoid form.
- Need to rush buildings and just make them go boom? A cat like or toy car suicide bomber like Goliath tracked mine.
- Secure a area already under control (road barrage)? Some sort of mini tanks with appropriate weaponry should be perfect.