I still think Last Crusade had the best ending, and the problem with that is that it makes Indy 4 seem like an afterthought.
Even the title, "The LAST Crusade" had meaning. It wasn't just the "last crusade" for the grail, it was Indy's "last crusade" (or so we thought.) And yet, now there's going to be another.
There were a few things I liked about Last Crusade's ending.
1. As Lucas has done throughout the series, he rehashed yet another old cliche... "riding off into the sunset." It was the perfect ending to a series that thrived on making old ideas seem fresh.
2. It represented Indy's growth as a character. In Raiders, he was a mysterious loner (at least until he reached the University), but by the end of Last Crusade, he is no longer a mystery or a loner. I think the ending showed this pretty well. He'd settled down somewhat, and his relationship with his father has reached common ground. As he rides off into the sunset, he's with friends; stable, long-term friends (unlike Willie or Marion). As I said in a previous post, Indy has grown from being a mysterious loner in search of "fortune and glory" to being everyone's favorite uncle.
It'll be interesting to see what happens in the next one. Will his character degrade to what he was back in Raiders? (nothing against the dark, mysterious figure, but it would be a step backwards in his life.) Or will he find more stability, companionship and happiness beyond searching for lost relics?