Which one is correct? The way I see it, the subject is 'the thing' which is singular thus it should carry 'was', no matter what follows. However, there seems to be a lot of people who use 'were' instead. Is 'the people' the subject in this sentence?
Your point is right. The correct way is
The most amazing thing was the people who...
I tried to think about the other specific possibilities. Sometimes we use "were" in the case of the subjunctive mood, however, it is used after "if, as if, wish, suppose". Here is not that case except if it is like
(I wish) the most amazing thing were the people who...
"I wish" was not said, but meant.