Here's a fairly exhaustive comparison between three contenders for marathon GOAT, at least if we restrict our attention to after 2000.
Haile Gebrselassie
Top time: 2:03:59
Avg of top 3: 2:04:26
Avg of top 5: 2:04:56.6
Avg of top 7: 2:05:17.1
Avg of top 10: 2:05:40.7
Avg of all* (12): 2:06:12.9
* I'm not counting DNF's
WR's (2): 2:04:29, 2:03:58
Wins (9): Amsterdam (05), Fukuoka (06), Berlin (06, 07, 08, 09), Dubai (08, 09, 10)
Win Streak(s): 1, 2, 6
Longest win streak: Berlin 07, Dubai 08, Berlin 08, Dubai 09, Berlin 09, Dubai 10 (6 wins)
Wilson Kipsang
Top time: 2:03:13
Avg of top 3: 2:03:26
Avg of top 5: 2:03:45
Avg of top 7: 2:04:08.7
Avg of top 10: 2:04:53.8
Avg of all (12): 2:05:32.3
WR: 2:03:23
Wins (9): Frankfurt (10, 11), Lake Biwa (11), Honolulu (12), London (12, 14), Berlin (13), NYC (14), Tokyo (17)
Win streak(s): 8, 1
Longest win streak: Frankfurt 10, Lake Biwa 11, Frankfurt 11, London 12, Honolulu 12, Berlin 13, London 14, NYC 14 (8 wins)
Eliud Kipchoge
Top time: 2:03:05
Avg of top 3: 2:03:32.3
Avg of top 5: 2:03:46.6
Avg of top 7: 2:04:05
Avg of all (9): 2:04:45.4
Wins (8): Hamburg (13), Rotterdam (14), Chicago (14), London (15, 16), Berlin (15, 17), Rio Olympics (2016)
Win streak(s): 1, 7
Longest win streak: 7, everything but hamburg.
The way I see it, Wilson Kipsang may abstractly have the best profile for GOAT. He's set a world record, is tied for 6th (4th if you exclude the Boston times) fastest all time, his 4th performance is the 16th fastest all-time, he won 8 marathons in a row, 4 of which were majors, and has won 9 total marathons including 5 major victories.
Unfortunately for Wilson, Eliud's career is nearly as impressive, better on account of consistency, and he's bested Kipsang on 3 of the four occasions they have met. No, he does not (yet?) have a world record, but his 2:03:05 clocking in London is the 3rd/4th fastest all-time, and anyone who watched the race knows it very well could have been a 2:02:5x performance [if not faster...]. His win-streak of 7 is nearly the longest anyone's has been, and is arguably the most impressive in terms of the wins, including 5 majors and one olympic victory. Moreover, while I didn't want to argue with the 2:00:25, i think it's undeniable Kipchoge would have beat any human that day and so his streak could easily have been 8. Of course if we are going to argue hypotheticals, it's worth noting Kipsang might have won 10 in a row were it not for coming up against Kipchoge and Bekele, two of the greatest of all time.
Haile's record is undeniably impressive, and it's true that his slower across the board times might be more due to the era he was competing in. Setting the WR twice is unthinkable in the modern era, although Kipsang has come mighty close. However, across the board his performances average almost a minute or more slower than Kipsang and Kipchoge's. I just don't think it's reasonable to rank him higher than them on that account. On the other hand, winning Berlin four years in a row is another achievement that seems unthinkable in the modern era.
In my opinion the current standings favor Kipchoge as GOAT, but there is certainly room for compelling disagreement. However, if Kipchoge sets a WR, or if he extends his win streak to an unprecedented 9 wins, I think that there can no longer be any argument. Really I think that if he achieves a sub 2:04 win in London this April, the case for GOAT is settled, but again I can see some people holding out unless a WR or truly unthinkable win streak comes about.