The movie is fine, but it's not one the Top 100 American movies of all time
According to the American Film Institute, The African Queen is one of the Top 100 American films of all time:
https://www.afi.com/100years/movies10.aspx
I just got done watching the film for the first time. It was okay, but at the end I felt like it really didn't amount to much. After a couple of scenes to establish the characters, nearly all of the rest of the movie is just two people drifting down a river, encountering normal obstacles and then falling in love. There's not much more to the story than that.
It's not a bad movie, but I wouldn't call it a great one either and certainly not worthy of AFI's list. In my opinion at least it shouldn't be on there at all and it definitely shouldn't be ahead of films like Raiders of the Lost Ark, Unforgiven or Saving Private Ryan.
If this exact film was made today with contemporary actors it would probably end up with a 65% on RT or something.