To me, this ties into the other thread about track improvements. It is impossible to compare hurdlers who run on modern tracks with those who ran on cinders. I can't help thinking that Harrison Dillard belongs in this field. An incredibly smooth hurdler who also fast enough to win a 100m Olympic gold medal.
That having been said, I can't see him keeping up with anyone on that list. It's just one of those imponderables.
If you had a 9th lane, I'd give it to Greg Foster who was, after all, a three-time world champion. Jason Richardson is another world champion deserving of consdieration. At one time I thought Rod Milburn was the ultimate hurdler, but I can't see him beating either Foster or Richardson.