Undoubtedly Raiders is the best film out of all 3. The opening sequence, the scene when they are digging into the Well of Souls in the sunset with Indy's silhouette pacing the dig site and the diggers singing, the melting Nazi faces, almost every scene is just classic.
That said, Last Crusade is tied with it in my book for pure enjoyment, one liners, chemistry between Ford & Connery, and expansion of the Indy universe without getting out of hand (ahem, Star Wars).
Temple of Doom is enjoyable, but overly cliched and silly at many times. Don't get me wrong, if it comes on TV, I'm sure to sit down and watch it through...I can't say that about many movies. The problem is when it's shown as part of a trilogy marathon, the others just outshine it so much, it makes it seem worse than it really is, IMO.
Hopefully the new flick doesn't let us down...