I think that TTT was probably the closest to the book. The only major changes I can think of are the attack of the Wargs and the elves at Helm's Deep (stupid elves, go away). Then there were some more minor things (no less annoying but not as important) such as the addition of Arwen where she is not needed and Faramir's personality change. I think there were probably less changes than in FotR or RotK, especially since they put the end of book TTT into movie RotK.
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