I think Raiders of the Lost Ark still stands on its own as being by far the best of the Indiana Jones movies. I think Last Crusade is also very good and I even enjoyed Crystal Skull. While I still liked Temple of Doom, it's the worst of the four for me. I thought it started out good but it just went downhill after they found themselves in India. There's still a 5th Indiana Jones movie planned and I hope they do a better job with it than with the last one.
It's funny how your sense of time changes as you get older. The first three movies came out when I was between 6 and 14 years old. The time between the first and the third seemed like an eternity but it was only about 9 years. On the other hand, I cannot believe it has already been 11 years since Crystal Skull came out...
Wait What! 11 years since Crystal skull! Man, I just checked...it's from 2008! Time sure flies.