Floris Evolved and Gekokujo that Steel recommended are good choices. Floris is basically the best Diplomacy-wise while still being the base game. Gekokujo is a great Medieval Japan total conversion.
I would say the best total conversion of M&B is
Prophesy of Pendor. There's an insane amount of work put into it (It could have basically been a stand-alone expansion like Fire and Sword) and it never gets boring.
However, it's easily the hardest mod out there so I never recommend it to anyone unless they already have a few good hours into the vanilla or basic modded game.