Diplomacy is a mod for vanilla, and its one of my favorite.
I think Prophecy Pendor is one of the best I've played in terms of content added and how smoothly it runs.
I've heard a lot about the 2 Lord of the Rings mods but I have not played them.
Also check out Perisno and Warsword Conquest. Perisno reminds me of Pendor, and Warsword Conquest adds many races from lizards to skeletons, and has some of the coolest looking Knights (of Bretonnia) I have ever seen. Both also have great soundtracks in my opinion.