Paul Tergat.
His track career happened to take place during the Gebrselassie era.
Tergat came 2nd to Gebrselassie in every World Championship and Olympic games over 10,000m ranging from 1996 to 2000. He also came 3rd in the WC in 1995. That makes 4 senior global silver medals and 1 bronze in the event. This happened to take place during one of the greatest championship race win streaks in history. Haile took gold in 6 straight major championships starting with the WC in 1993 and ending with the gold medal in Sydney 2000.
Haile admitted in an interview, that it was unfortunate, because if not him, Paul Tergat would be considered the best 10,000m runner of all time (or at the very least, the greatest before Bekele or Farah came into the picture).
Tergat's 5 years on the track coincided with Haile's incredible 7 year win streak
Tergat is third on the all time list for 10,000m with a personal best of 26:27.85 which he ran on August 22, 1997 to break Haile's then standing record of 26:31.32. Only to have Geb run 26:22.75 the following June.
He was successful in XC but I don't know many people who argue against the track having higher priority over the grass.