Definative Medieval Total War Mod?

[Martok]

What Ajax said! I personally have XL installed on my game, but I've heard almost nothing but good things about the BKB mod and MedMod as well--it really all depends on what you're looking for. You may want to visit the Engineering Guild, which has a pretty good despription of all the various mods. You can also check out this thread: https://forums.totalwar.org/vb/showthread.php?t=53100. It does a pretty good job of outlining the differences among the 3 mods, along with discussing the pros and cons of each one.

[Mithrandir]

Forward notice : No mod-bashing.

[Byzantine Prince]

The BKB mod is the most extensive and well-researched one. It does however have it's bugs. The XL mod is a lot lighter but still very enjoyable.

[Martok]

Forward notice : No mod-bashing.

Indeed. That's why I referred him to the already-existing thread. Otherwise we're all just going to wind up repeating ourselves!

[econ21]

The short answer is no, there is no definitive mod in the way that RTW has RTR, IMO. (Although I suspect RTR will lose that status too when EB is released.)

The mod I am most familiar with is the MedMod by WesW. The premise of that mod was to increase the challenge of the game - ie gameplay - rather than prioritise realism a la RTR. However, it probably has made MTW more realistic along the way (e.g. by restricting recruitment of core troops to your homelands; an idea RTR also adopted).

I'll have to try some of the other mods, some of which seem to emphasise historical flavour. But none of them seem to be made by the large teams of historically minded modders that you see with RTR and EB, hence they are arguably less "definitive" than RTR currently is.

Personally, I think MTW vanilla is about as historical as RTW vanilla, so I doubt staying with that would give a more realistic game than a popular mod.