It depends on which version you are running. Original MTW needs patch 1.1 and does not have many mods. Viking Invasion requires patch 2.01 and for this version most of the mods are made. Personally, I run MedMod v2.14, because it does not change the game but makes it thougher by programming the AI to invest more in trade and farming. This prevents the AI from going bankrupt (like it usually does in MTW) and allows it to build more powerful troops. If you want a different game but still a medieval one, there's MedMod v3, Kekvit Mod, BKB's Supermedieval and Medieval XL. If you want an entirely different experience, then Napoleonic TW, Hellenistic TW, Shogun for MTW:VI are all excellent choices.