Over the course of the last century, general relativity has been tested to remarkable accuracy and has withstood critique each time. General relativity was such a giant leap forward that it is arguable that had Einstein not formulated the theory, it may have remained undiscovered for a very long time.
Like any major discovery in history, Einstein's did not happen in a vacuum. The were actually quite a few people thinking about relativity at the time. Off the top of my head I believe David Hilbert, Henri Poincare, Hermann Minkowski and Hendrik Lorentz were all investigating in the general direction of gravity's effect on electromagnetic waves and curving of space time. A couple of these dudes might even be able to stand as contenders for the "smarter than Einstein" competition if such a thing weren't patently absurd. Hilbert in particular is immensely respected by mathematicians and occasionally physicists. It also wasn't even the first time relativity was incorporated into scientific theory. Galilean relativity had been considered for many years (since Gallileo?) and was incorporated into the theories of Newton. The point is, I don't think anybody has ever possessed incomparable genius. Einstein wasn't some kind of scientific hercules and I'm not even convinced it would have taken even one decade for somebody else to propose relativity let alone several. I leave you with this, it's a bit long but well worth the read if you can get through. Then you can impress all your science friends by claiming, "Yeah, Einstein wasn't that great," and being able to argue the point convincingly. https://en.wikipedia.org/wiki/Relativity_priority_dispute#The_candidates_for_credit P.S. I have nothing but admiration for old Albert, I'm just not into hero worship.
Maybe without Einstein there would have been another Einstein a year later? Maybe there would have been a big splattering of advances across a ton of scientists due to some breakthrough somewhere? Maybe having such a diversity of opinion in figuring out relativity would have caused a larger growth in knowledge than if we had Einstein around. Maybe if Einstein hadn't been born the earth would have fallen out of it's orbit due to a different mass distribution and we would all be dead! We don't know, those things didn't happen.