Which one is correct?
The scientist is really an Einstein of our time.
The scientist is really the Einstein of our time.
If you think he is uniquely qualified to be compared to Einstein, then use "the."
If you think there are other scientists who are just as revolutionary in their thinking, then use "an."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.