Day 5 of the non-D&D blog challenge
This is a loaded question, because I think it has a lot more to do with who was first out of the starting gate than anything else. Any game that amassed the following that D&D had, was a reasonably decent game, would have made it and crowded out the other contenders. D&D was the one that would become as big as D&D in that case. It set the stage for what was expected of a role playing game - had it been a Pride and Prejudice inspired game, we'd have a hobby with very different priorities than we have today.
With that being said, given society and culture at the time I'd think it would have to be another fantasy-type rpg that would have become as big - and I'd put my finger on Runequest over any of the other contenders such as Tekumel. Runequest was accessible, had its own style, and could have easily become as popular as D&D became.