Finding the best basking light is always a matter of trial & error because no two situations are identical.
IMO it's best not to think about wattage because it can mean very different things depending on whether we're talking about a household incandescent bulb, incandescent flood, or halogen flood (and the type of fixture/reflector).
Instead, find the bulb that gets you the basking temperature you're aiming for, at a distance such that the chameleon won't burn it's little toootsies if it happens to ceiling-climb. Once you've established the temps & distance, you should be good for that enclosure unless something else changes.
I would avoid "spot" lights of any kind, as nightanole mentioned. One way I remember is that 'spot' lights produce hot spots; flood lights flood the area.