1. The Lord of the Rings: The Fellowship of the Ring (If I must choose only one...) - Duh.
2. The Silence of the Lambs - Classic performances, great soundtrack, amazing writing and dark scenes.
3. Finding Nemo - Best CGI-animated movie out there. Soundtrack is wonderful and it brings back memories.
I can only give you a top three because after that it gets really ambiguous. I still have a humungous backlog to get through.