At the moment I'm playing the Floris expanded mod pack with native troop trees and combat animations (god how i dispise the "enhanced" animations) which basically is a "best of" all the single player mods rolled into one. Definitley worth a look and relativley easy to tweak stuff if you don't like it.
If its not that its bog standard native with the diplomacy mod (a must!) I'm not a big fan of the total converstion mods set in the "real world" of, say 1257, or whatever and would rather be in Caladria. The one mod i'd really like to try would be 1886 but sadly that is only for the original Mount and Blade.
The one to watch is Warhammer: Grim Age. Made by Russians and in a very, very early stage of development this looks to be the most promising of any of the mods I've seen. Check it out in the pioneers guild on the TW forums.
A good single player mod is a must in my opinion and in the 600 or so hours i've played on M&B:WB about 250 of those are in single player. Very helpfull when its 3 a.m and the EU servers are dead!
