Raiders of the Lost Ark has the best balance between action, romance, and comedy. I still love the boulder at the start, the fight at the bar, bringing a gun to a swordfight, the kiss on the ship, and the finale with the Nazis getting obliterated.
No other Indiana Jones quite struck that balance.
My full ranking:
1. Raiders of the Lost Ark
2. The Temple of Doom
3. The Last Crusade
4. The Dial of Destiny
5. Temple of the Crystal Skull