I think that they have to be viewed together to be fully appreciated. FOTR is actually my favorite of the three, but on its own, does it stand out as the greatest movie ever made? I don't know. Lists like the AFI Top 100 honor single films that have stood the test of time, and LOTR is too recent to have done that AND it's a series. That makes ranking it difficult if you are talking about individual movies--ROTK was powerful because of the two that came before, and it shouldn't be singled out for greatness at the expense of the others (like the Academy Awards unfortunately did). Many would agree with you if you call it the greatest series or the greatest trilogy of all time (even the Godfather fans would shut up when you bring up part three of that beloved series). But if you say that they are the greatest films of all time, that means that individually, each movie is number one, two and three on the top movie spots. (And I would personally rank them FOTR, ROTK, and then TTT.) If I worked out a list of my top twenty (or probably even top ten) movies of all time, all three would be on there. But I don't know if they would all be in spots #1-3.
So I will just say that individually they are among the greatest movies ever made, and that together they form the greatest trilogy/series of all time.