Short Answer: Indiana Jones and the Staff of Moses
The best films in the series are the ones that involve a quest for a major Biblical artifact. Now that the Lost Ark and the Holy Grail have been found, there are really only two more major miraculous objects in the Bible:
One is the breastplate of the High Priest. It contained gemstones, each representing a Tribe of Israel. When the High Priest posed a question to it, stones would light up in a sequence which revealed the answer, even if it was about events yet to occur. But blinking stones doesn't make for great cinematography.
You know what does? Biblical plagues. Water, gushing from a rock or rising to form solid walls. The ebb and flow of two foes in combat.
And what causes those? The Staff of Moses, which he receives at the Burning Bush. (Yes, there is a video game in which Indy goes after it; I discuss that later.)
God uses it (Exodus, 4:2) to show Moses that He can do miracles. By turning it into a snake.
And you know who hates snakes? Indiana Jones.
The movie: The Staff of Moses may be found, and Indy rushes to obtain it before the Nazis do. Over the course of the movie, various plagues are conjured with it, water is brought forth from a boulder, and a sea is split in two. When the Staff is raised, the good guys win in a fight, as long as it stays raised.
Then comes the climactic scene in which the Staff is cast down by, let's say Belloc, and turned into a snake that starts slithering toward Indy.
Indy has to overcome his greatest fear-- "Why did it have to be snakes?!?"-- and grab the snake, trusting that as soon as he grabs it, it will turn back into a wooden stick.
He conquers his deepest fear, one we have known about for the whole series... and saves the world.
This plot caps off the series by having Indy complete his greatest quest, round up the last viable Biblical artifact*, and conquer both his greatest external enemy-- Belloc-- and his greatest internal enemy: his fear of snakes.
There is a video game in which Indy seeks the staff, there called The Staff of Kings for some reason (Moses was many things-- prince, shepherd, prophet-- but never a king). In the game, he uses the-- once he finds it, all the way at the end-- to part some water, then shoot a beam from it that takes out a blimp. Which... was not mentioned in the Bible. Also, at one point, its tip glows to light the way because... Gandalf?
At the very end, it turns into a snake and Indy discards it. He doesn't even say the line! It never gets to call down plagues in the game, either, that I can see. Yes, Indy hates snakes, but he would never toss aside a priceless artifact.
The game seems to have been discontinued, but clearly the makers of the series know of the staff and some of its powers. So this is a definite maybe.
Except Part 5 is being worked on as I type this, and it seems like they are go for time-travel instead. Whatevs.
But after this one... yeah, let Harrison Ford retire the character.
Then either continue it with another actor... or build a series around his father, Henry Jones, and his early adventures. I bet a lot of actors would want to play a guy who grows up to become Sean Connery.
Or... it may be OK to retire the character altogether. The idea of an American taking treasures away from natives to stick them in a museum is one that may need to become, well, a historic artifact that lies untouched.
*Does anyone else remember the brass snake from Numbers 21? Well, since it's a snake ON a staff wielded by Moses, we can throw in it as a bonus artifact, or maybe a red herring on the way to the real snake-staff...
