EL G has been the most successful due to running in a period where they ran World Champs every 2 years and when EPO use was at its most widespread and undetectable. The only athlete that can compare to number of world titles and fast times was Morceli, because he was competing in the same era.
If they'd had World Champs every year since WW2 then Elliott would have won a couple, Snell, Ryan and others.
Ovett won 2 World Cup golds 4 years apart and these should be viewed as equivalent to World Champ golds if you include these in any assessment.
Coe would probably have won 2 in 79/81 and Cram would likely have added to his inaugural World Champ gold in 83 with another in 85 had they been held biannually.
Any real assessment of greatest ever has to look at the things that were available to all athletes ever, and that means Olympics and world records first and foremost. The opportunities to win outdoor and indoor World golds just didn't exist prior to the last 20 years or so.
With this in mind there are 4 athletes whose actual achievements put them above the rest, namely Elliott, Coe, Morceli & EL G.
They are the only ones to have won Olympic gold and set world records at both 1500 & Mile.
Elliott won 1 Olympic gold and set 2 1500 world records and 1 Mile wr.
Coe won 2 Olympic golds and set 1 1500 wr & 3 Mile wrs.
Morceli won 1 Olympic gold (after failing when favorite in 92) & set 2 1500 wrs and 1 Mile wr.
EL G won 1 Olympic gold after 3 attempts. He set 1 1500 wr & 1 Mile WR
Snell, Ryun, Ovett, Cram & Lagat are probably the next tier of greats, but all are either missing an Olympic title or a WR in one or other of the 2 distances.
After Olympic medals & world records, then of course the number of years at the top and the number of fast times come into play, as do other medals in minor events. But even these are subject to the time in which they competed. Running fast times behind rabbits was the vogue in the 90's, while in earlier periods there wasn't the same desire for fast times every week.
For me it has to be between Coe & EL G. Coe, because he is the only man ever to win 2 1500 golds and had to beat 2 other greats, Ovett & Cram to do so, and because he broke more 1500/mile wrs than the other 3, despite remaining primarily an 800m runner.
EL G because of the number of years he was ranked no 1 and his number of fast times. However, people do forget that he was beaten twice on the biggest stage of the Olympics, which must go somewhat against him. I also believe his times were enhanced by peds, as were many he competed against. He was clearly the best of the bunch in the 90's, but without EPO I think we can add at least 2 seconds to his best times at 1500 & mile.
Therefore final list based on Olympics, WR, ranking points, number of fast times, longevity, win/loss record against peers....
Coe
EL G
Morceli
Elliott/ Ovett ~( yes I know he didn't win Olympic gold at 1500, but I put him
alongside Elliott due to more wrs and 45 unbroken wins)
Cram/Ryun
Snell
Walker
Lagat
In 2004 all the mile world record holders as far back as Bannister met up for a special commemorative dinner. Ovett was the only person who didn't attend. They asked all the record holders there, as well as other athletic journalists, who they thought was the greatest miler of them all, and the athlete with the most votes was Coe. This was obviously in the May before EL G finally won his Olympic gold, so maybe the result would have been different a few months later. But I think the verdict of your fellow peers is probably the most important.