It's according to your preferences although there are quite a few ones that find themselves on the top of everyone's pick:
Sands of Faith a mod that's set between the Third Crusade and the Fourth (I'm not sure on this one, I'm not that good in history stuff but I think it is? Also, I know the fifth was the Children's Crusade so it's between the Third and the Fourth in my perspective)
Prophesy of Pendor a great mod which has a deep lore and tonnes of new features. You can have custom troops, recruit powerful units into your army and you have to fulfill specific tasks to be able to conquer the whole land. There's also random events like a large army spawning every few times with 400+ soldiers. Some of them are led by unique NPCs which you can get unique items from.
Warsword Conquest is a Warhammer Fantasy Battles based mod. If you ever heard of Warhammer Total War by CA, the mod is based around the same universe. There's the Orcs, Skaven, the Chaos (literally demons), the dwarves, the elves, the Vampire Counts, the Empire, the Bretonnians and much more. Head over to
www.whfb.lexicanum.com to learn more about the races and history of the universe.
Floris Mod Pack is a compilation of mods first used in the AAR (after action report), "Me, Floris" by monnikje. It adds tonnes of new stuff, replacing the old troop trees, adding over 500+ items, more diplomatic options when you are a king/queen. There's a lot of new stuff and FMP has been used as a base for other mods like Brytenwalda.
Tohlobaria is a fantasy mod which is a total overhaul. New map, new factions and of course new features. It's very underrated in my opinion and I haven't seen anyone talking about Tohlobaria though.
A World of Ice and Fire and
A Clash of Kings are two mods based on the (more popular) TV Show which in turn is based on the novel series "A Song of Ice and Fire". Both have different features which I'd like for you to discover it by yourself.
Blood And Steel is a Native-based mod with new features that focuses on strategy rather than the usual "get recruits, train them until strong and charge in" tactics that we used to. It has new units as well and armies are well in the thousands. Don't worry, it has been stated that it has been configured to be able to run on decent computers.
Phantasy Calradia is basically Calradia with magic, elves, dwarves, wizards, drows and the like. It's fun because you can throw fireballs at your enemy and hear the Drows go, "Ayayayaya!". It's funny tho to hear the voice lines xD.
Gekokujo is a mod based on feudal Japan during the Warring States period. New scenes and features. It's a large map that even I couldn't conquer the whole thing.
Sword of Damocles: Warlords for Warband adds a LOT of new factions. It has new features which include spreading your religion by building its churches/monastery and such in towns. Recruit a vassal from a faraway land and more. It had a lot of factions and I'm not sure if you can conqier them all since vanilla factions also exist. The map is huge as well.
