One thing fascinating about running is that WR holders are not necessarily the best - we still argue about what this or that runner could have run if not injured, if that rabbit had done his job better, if the weather is great, etc.
consider a situation where all the great runners could deliver their maximum potential in a race. A perfectly rabbited race, in great weather conditions, in the best shapes they could be in.
What times could they have run?
and if all the runners could have run their bests, who would own the WR of middle/long distances?
is this the same of asking who's the most talented runner ever?
maybe yes - and for that I'd answer Daniel Komen. For me he is the supreme 3k/5k runner.
For middle distance - 800/Mile I think of Steve Ovett. I know a lot of americans will mention Jim Ryun and I agree he is a good contender.
For long distances, I will mention one Brazilian - as I am brazilian I might be partial, but the Marathon WR of Ronaldo da Costa in 1998 is probably the most underrated WR of all time. Running the second half in 61 and crossing the line as nothing had happened was impressive. I know him personally and can tell the guy was a monster, had he taken better decisions in life, he could have had a consistency career of record-breaking.
So for me,
800-mile - Steve ovett
3000 - 10.000 - Daniel Komen
Half - Marathon - Ronaldo da Costa