Wow, I have an entirely different ranking than anyone so far:

The Last Crusade

Raiders of the Lost Ark

Kingdom of the Crystal Skull

Temple of Doom

I actually saw The Last Crusade first of all the films, and as it opens with Indy's backstory, I always just assumed that it was the first in the series (the Ark reference confused me at the time!). People say it's weak, but I thought Connery was excellent as the father, and the entire Grail quest is enthralling. And the music!

Anyway, If I'd watched Raiders first as a kid, I probably would have ranked it highest. But as I was spoiled by the effects of Crusade, this didn't have as big an effect on me as Crusade.

I went to see KOTCS in the theater, and distinctly remember giving the above ranking to all the movies upon leaving. There are some atrocious moments in the film, like Shia swinging on vines, but overall it was different enough from the others to be interesting, something that I cannot say of Temple of Doom.

Temple of Doom is the pits. I don't care about the opening dance number, the inflatable raft escape is laughably absurd, the magic stones are parochial and dull, Short Round is annoying, Willie Scott is more so. It is thoroughly unpleasant throughout, gross for the sake of being gross, cruel (child slavery?!) for the sake of being cruel, and justly deserves the scorn shown to movie prequels on these forums.