I would say Medieval 2.

Napoleonic Era: Problematic -> Gunpowder units were the worst part of MTW (IMO). They would fight on new terrain with the introduction of gunpowder, while

MTW 2 -> familiar ground, plus the experiences of RTW with the new 3D engine.

Good chance that this game would not only feature one of the imo most interesting periods, but also improved gameplay free of major bugs and the gripes of a completely new game / system.