Fellowship, because it was a new and exciting glimpse of how P.J. saw Middle Earth. It started to go south about the time Arwen "caught a ranger in the wild off his guard"
The other two movies, while having their moments of brilliance in scenery and acting of specific scenes, really just amplified the digressions. TTT was a big attempt to band-aid it all together, and RotK was too hurried where it shouldn't be, while time was wasted on too many anti-climatic false endings.