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.