QFT.Alien 3 has a great ending. I was very moved by Ripley's heroic sacrifice, and nothing the haters say can ever change that.
Alien 3 has some genuinely moving emotional moments, more than the other films put together, I'd say.
The cremation scene is extremely powerful, with the intercuts between the falling bodies, the death of the dog and the birth of the alien, and Charles S. Dutton's benediction. This scene has moved me to tears.
Then, Ripley's plunge into the cauldron. I find that scene uplifting, as even in her final act she still fights against the uncaring universe.
I cannot think of a comparable scene in the other films that hits in quite the same way as these two do.