I haven't seen every single Agatha Christie adaptation, of course, but I know that "Murder on the Orient Express" is considered by most fans to be the best of them all. I believe that "Death on the Nile" in a vastly superior film, for the following reasons:
1) Ustinov is a much better Poirot than the cold and dislikable Albert Finney. Ustinov combines intelligence and sweetness in the role, and the scenes where he displays Poirot's vanity ("I leave luck to others!") are hilarious.
2) The logic in this movie is flawless and everything is CLEARLY explained, while in "Murder in the Orient Express" Poirot reached many ARBITRARY conclusions. You won't guess either ending, but it takes a brilliant person (like Poirot) to guess the ending here, while it takes a magician to guess the ending there.
3) "Murder on the Orient Express" is somewhat claustrophobic, while "Death on the Nile" contains many more scenes filmed in beautiful natural locations. Visually, it's a much more pleasing film.
"Death on the Nile" has only one serious flaw: its overlength. But it's still an unpredictable, highly enjoyable film, and the scene where Poirot reveals everything is so perfectly done that you'll want to watch it again and again.