Fellowship of the Ring for numerous reasons, though the trilogy itself is God tier.
Fellowship because (IMO):
Best death scene in cinematic history (Boromir)
The Fellowship all together
the least amount of Legolas
I really love the trilogy. But Fellowship, whether it is the theatrical or the extended cut, is more enjoyable to watch multiple times. Also, if you watch the theatrical cut you aren't really missing crucial character development scenes. I mean, there is just more of what is already great. I still cannot believe they did not add the Boromir/Faramir scene to the theatrical version. It adds so much more character to Faramir that I feel without it, he doesn't have much.