It's Solid Snake until proven otherwise, and there's really no reason to think it's anyone else. Liquid is worm food (his body, anyway), Solidus was looking that old back in 2009 (and had his spine severed), and Big Boss lost his right eye, not his left (and was burnt to a crisp).

And we don't know for sure that Solid is actually missing an eye, there. The rad cyberpatch could just be some high-tech sensory device--and a sly reference at the same time.