K6-2+, assuing your super7 board's bios supports it (check the manufacturer's site for bios updates tho), many k6-2+'s are hitting 550-600mhz, which aint too shabby, at least according to reviews and some users in 3dnow.net's messageboards.
Failing that, K6-3 450 if you can find one around, although they arent exactly the most overclockable cpu's out there.
Leaves the k6-2 533/550's in last place.