I don't play this game anymore for various reasons, but my vote goes to Divide et Impera. That's the best mod in terms of historical accuracy. One can feel it's a labour of love of history buffs. It brings so many new features (e.g.population system, supply system) which make the game a lot more strategic and thus completely different than vanilla.
There's problem with balanced difficulty level throughout the whole campaign in this mod though, making early game extremely difficult but getting a lot easier as you progress in the campaign, but that's rather a general problem with TW games.
Once you played DEI, you will never go back to vanilla.