I'd say, Webb, at his best, would beat Ritz at every event that is contested on the track. Ritz would beat Webb in 12K XC and the road races 10k+.
Ritz ran 27:22 big deal. He lost to Webb when Webb ran 27:34. You site that Ritz could have gone faster - but I'm assuming you must not think Webb can? Webb also ran his 13:10 WAY back in 2005 after a season training for the 1500m.
If you are going to ask the question, "at their best," you are basically asking who is the biggest talent. If that is the question, I'd choose Webb in every race from 1m-10,000m on the track.
But in terms of who has had the best career, probably Ritz. He is a
3:40 (?)
8:11
12:56
27:22
60:00
2:09:55
3x Olympian
NCAA Champion XC
3rd USA JR XC
3rd World Half
6th place WCs 10k
9th Oly Marathon
Webb:
1:43
3:30
3:46
8:11
13:10
27:34
1x Olympian
3x USA 1500m Champion
3:53 HSR
7th WCs
9th WCs
#1 World Ranking in 1500m 2007
#2 World Ranking in 800m 2007
So it's between Webb & Ritz.
Hall has been the most consistent, but he has really never been that competitive on the world stage. He is a consistent 2:08 guy. He is generally 2min back from the winner. He has never really been a definite top 10 guy at any point in his career.
Most marathons only have ~8 quality guys max - he routinely gets ~5th or so. But during that same month, other guys will run 2:04/05/06/07 all across the world in other races.
With that said, I'd say Rupp has ALREADY had a better career than all of them.