In my mind, it comes down to Zatopek, Haile G, and Kenny B. I'm going to eliminate Zatopek despite his Olympic triple just because he was so much slower than the other two--he dominated his generation but at some point he just ran too slowly to be compared to the others.
Then its Haile vs. Kennenisa.
In Geb's court: multiple 5k, 10k, M world records. The 5k and 10k world records once he was done with them were WAY faster than when he started. The general dominance he had over everyone early in his career is something rivaled by nobody. (see:
http://www.youtube.com/watch?v=GjSkL1MGWlY&t=9m30s
). Marathon domination that Bekele doesn't have. Finally, indoor cred: 2nd fastest 1500 of all time & mulitple golds that bekele doesn't have.
In Bekele's court: has run (marginally) faster than Geb / current 5k / 10k WRs. Faster marathon debut (jury's still out on marathon career). Olympic 5k gold that Geb never had. Most importantly, a SH*T ton of XC world golds, which Geb never had success in.
I would say if Bekele runs under 2:03:30, he is the GOAT. Otherwise it's a judgement call--but I would give it to Gebrselassie.
Honorable Mentions: Paavo Nurmi, Abebe Bikila, Lasse Viren, Sebastian Coe, Hicham El Guerrouj, Bernard Lagat.
NOT Mo Farah until he does something other than win a tactical championship. XC, marathon, running a WR--he needs something.