Three books have come out in the last year or so by or about US runners who lost the opportunity for an Olympic medal due to unfortunate circumstances: Craig Virgin ("Virgin Territory"), Eddie Hart ("Disqualified") and Mary Decker Slaney ("Olympic Collision"). My ranking for most unfortunate in that regard would be:
1. Jim Ryun 1500m (1968--at career peak--had to run at Mexico City altitude against a Kenyan after recovering from mono, then 1972 tripped in a prelim)—got a silver, but had a good shot at two golds
2. Mary Decker Slaney 1500/3000m (1980 US boycott, 1984--at career peak--tripped in the final)--had a good shot at two medals, likely one gold
3. Bill Rodgers marathon (1976 injured, 1980-at career peak-US boycott)--good shot at two medals, maybe one gold
4. Craig Virgin 10,000m (1980-at career peak-US boycott)--good shot at a medal
5. Eddie Hart and Rey Robinson 100m (1972 disqualified for arriving minutes late to a prelim)— Hart got a relay gold, Robinson nothing--each had a good shot at 100 medal, maybe a gold--
6. Steve Scott 1500m (1980-at career peak-US boycott) --good shot at a medal