1. Whitaker
2. Jones
Both had long stretches in their primes when they were unbeatable. Both were so fast that they never got hit in their primes, which made everyone wonder whether they had good chins or not. Late in their careers, they lost their speed and did get hit with big punches. Whitaker proved against Trinidad (the best puncher in the sport at the time) that he had a great chin. Jones, on the other hand, was KO'd by Tarver and Glen Johnson. That's the difference to me.
3. Hopkins
4. Chavez

I don't think you can rank Floyd, because he hasn't lost his skills yet, and he's still only 29, his career isn't near the end. If he's still undefeated at 34 or 35, then he's #1 on this list. If he loses in the next year or so, then he's no higher than 3rd or 4th, maybe lower.