Close between JK and MotS. But I have to go with the original JK. Tracking down all of Jerec's cronies while searching for the Valley of the Jedi and learning the way of the Jedi was so much fun. Lots of mercenary action, too. If only Force Jump wasn't so clumsy. Come think of it, if they remade that game using Raven's modded Q3 engine, it would mop the floor with both JK2 and JA.
MotS was great too, up until Dromund Kaas.