Raiders stands as my favorite movie of all time period, so it was the obvious choice for me. I doubt that I'll ever be so enamoured with a film in my life, primarily because its tied up with childhood nostalgia (well...I suppose Star Wars matches it on that level). Raiders is just such an economical and masterfully constructed action film, and I don't think its been equalled to this day...it really is a modern classic. Last Crusade is just a lot of fun, and has great action set pieces, my favorite of which being the Tank sequence...that's just fantastic.
I enjoyed KOTCS more than Temple of Doom as a whole...but... there were stronger individual scenes in TOD (the mine cart sequence being the standout), and I love its soundtrack as well. Crystal Skull was hopelessly stupid, but I also really enjoyed the fact that it drew heavily on fifties cultural references. It could have been much better I suppose, but my expectations were pretty low (I thought the whole idea of making a new Indy film was ill conceived to begin with), which helped my impression of the final film quite a bit. Harrison was fantastic in it, and that went a long way...it was great to see him as an older and more curmudgeonly version of the character, and he still pulled off the action beautifully.