Wilson Kipketer.
Coe is 3rd - he was a great 800m time trailer but not the best tactician over 2 laps with his only major win being the Euro in 86. He's also over half a second behind Kipketer in his ceiling performance, and broke 1.42 once vs multiple times for DR and WK. It's no slight on him at all as he's the best 8-15 guy ever, but he doesn't win this.
So the way this hypothetical race is decided is can Rudisha outrun and hold off Kipketer? He doesn't win any other way because he never won any other way in his entire career other than front running. Might seem like a simple answer because Rudisha actually has the WR and ran 3 times faster than Kipketer ever did but I'm not so sure.
I think Rudisha ran to his max in London. People say "what about drafting and having a good pacemaker etc" but in the 800m and it this level of human performance this is only a benefit if the pacemaker is essentially a clone of the runner they are pacing at get's the rhythm, tempo and execution absolutely perfect. What I saw in London was Rudisha getting to run the 800m exactly how he needed to in perfect weather with the perfect incentive. No critical section of the race was too fast or too slow - there was no insane 23.5 opener, cooling down to 48.8 at the bell (which is still too fast), he got to execute the way that was best for him and he did it to perfection.
Kipketers WR in Zurich on the other hand is one of the best performances in running history ever not just for the fact that it smashed a superb WR and is still the 4th best time ever, but because it survived a suicidal opening 200/400m (23.1, 48.2 that wasn't even close to optimal. We know this because a week later Kipketer ran faster worse conditions and atmosphere in Cologne "despite" going through 400m almost a second slower. So really on this night in Zurich, Kipketer could have run somewhere in the 1.40.6/7 range had he hit the bell in 49.0. I mean Rudishas agent James Templeton even said this to me straight up once - they were happy Kipketer "messed it up" (somewhat tongue in cheek obviously) that night so David could break the record in London.
So I don't see Rudisha, with the threat of Kipketer on his shoulder, being able to run away from him and Kipketers slightly superior form and racing pedigree (watch the 99 WC final to see how tough he was) being enough to just get by and take the win.
Just an opinion.