A lot of good reads on the topic, thank you. I'll add my two dinars.
To answer the OP's question, I would definitely say Vlandia. The reason? While it's supposedly been 210 years from the times of Bannerlord in Warband, the Swadians are basically the *same* faction. When I heard talk about Vlandia I would have expected a proto-medieval realm, not a fully realized kingdom with knights and sergeants, crossbows (that shouldn't have existed if this is supposed to be 210 years before Warband, and in a setting that's based on our own history) and cities and castles which designs are virtually identical to those seen in Swadia two centuries later.
There are other issues I can see (all IMO) with the other factions.
Sturgia looks a lot like the Kingdom of Nords and is losing a lot of its slavic flavor with each and every update while also being underwhelming on the field *and* on the walls.
The Empire(s) does a good job in looking a lot like what you would have imagined a pseudo post-separation Eastern Roman Empire to look like. Still, while I don't think there should be that much difference between the sub-realms within the Empire, there should be at least some flavor to it: different mechanics, perhaps? Different approaches to military campaigning, troop composition, kind of missions provided to the player maybe? I also cannot imagine them using crossbows this early, they'd probably be using javelins and skirmish more instead, or rely on large amount of light cavalry.
Aserai. I haven't fought them (or alongside them) long enough to have a strong opinion one way or the other... having met them on the field so rarely and in such small numbers I genuinely have nothing to say about their faction, so I will refrain from commenting ?
The Khuzait are a little bit extreme considering the time period we're supposedly in (I realize this is not *our* world, but it should follow its history quite closely) when it comes to armor and troop composition. Either give them a throng of horse archers, skirmisher and riders with light armor and nimble yet fragile mounts while cracking down a little on just how well armored their horse archers can be or make it more difficult for the AI to recruit vast number of high-end riders. The mongols weren't mostly armored horse archers even at the height of their empire, their strength was in the mobility and the numbers.
I've seen some great posts about Battania, too... and they said more than I probably could. Still, I think that (especially when it comes to game balance) Battania has a troop roster that's a little... weird. Alright, so... only nobles use bows on the open field. But... why? If they're supposed to be a skirmishing faction with a lot of hit and run, then let them have better AI and better thrown weapons (which have been nerfed recently, too...) instead of making them a strange amalgamation of butt-naked pict warriors and welsh longbowmen with a peculiar taste in clothing.
Considering in such a difficult time for Calradia food would be scarce and a LOT of people would be poaching, it'd make a lot of sense that a lot of battanian peasants would know how to hunt, on top of owning a bow to hunt with, of course.
Speaking more broadly, I think every faction in general should be more lightly armed and armored (especially armored.) Perhaps adding some diversity to the leather (cuir bouilli for example) armor selection and some heavier clothing would help alleviate that issue, alongside weaponry that would better suit the time period the game is built on. More diversity in the peasants' clothing would be nice, they all look a little too prim and proper when they're just mere recruits from a pre-medieval society. Also, the game should have wildly different mechanics for each factions when it comes to battling enemies, managing logistics and handling intrigue and politics. Khuzait should be able to automatically "siphon" some food from the lands they're trying to take, having so much fast cavalry and generally employ horde tactics when in superior number while harassing and skirmishing before retreating when the battle is hopelessly against them. The Empire should have lower morale when fighting another Empire-based faction, they could also have unique formation (that really goes for all factions though) and a deeper intrigue system.
TL;DR=More diversity for light armor, less heavy armor, bows for Battania, more fragile HA, less advanced weaponry, more interesting game mechanics which could give the factions more flavor. Also, sorry about the post being messy, it's really late here.
To answer the OP's question, I would definitely say Vlandia. The reason? While it's supposedly been 210 years from the times of Bannerlord in Warband, the Swadians are basically the *same* faction. When I heard talk about Vlandia I would have expected a proto-medieval realm, not a fully realized kingdom with knights and sergeants, crossbows (that shouldn't have existed if this is supposed to be 210 years before Warband, and in a setting that's based on our own history) and cities and castles which designs are virtually identical to those seen in Swadia two centuries later.
There are other issues I can see (all IMO) with the other factions.
Sturgia looks a lot like the Kingdom of Nords and is losing a lot of its slavic flavor with each and every update while also being underwhelming on the field *and* on the walls.
The Empire(s) does a good job in looking a lot like what you would have imagined a pseudo post-separation Eastern Roman Empire to look like. Still, while I don't think there should be that much difference between the sub-realms within the Empire, there should be at least some flavor to it: different mechanics, perhaps? Different approaches to military campaigning, troop composition, kind of missions provided to the player maybe? I also cannot imagine them using crossbows this early, they'd probably be using javelins and skirmish more instead, or rely on large amount of light cavalry.
Aserai. I haven't fought them (or alongside them) long enough to have a strong opinion one way or the other... having met them on the field so rarely and in such small numbers I genuinely have nothing to say about their faction, so I will refrain from commenting ?
The Khuzait are a little bit extreme considering the time period we're supposedly in (I realize this is not *our* world, but it should follow its history quite closely) when it comes to armor and troop composition. Either give them a throng of horse archers, skirmisher and riders with light armor and nimble yet fragile mounts while cracking down a little on just how well armored their horse archers can be or make it more difficult for the AI to recruit vast number of high-end riders. The mongols weren't mostly armored horse archers even at the height of their empire, their strength was in the mobility and the numbers.
I've seen some great posts about Battania, too... and they said more than I probably could. Still, I think that (especially when it comes to game balance) Battania has a troop roster that's a little... weird. Alright, so... only nobles use bows on the open field. But... why? If they're supposed to be a skirmishing faction with a lot of hit and run, then let them have better AI and better thrown weapons (which have been nerfed recently, too...) instead of making them a strange amalgamation of butt-naked pict warriors and welsh longbowmen with a peculiar taste in clothing.
Considering in such a difficult time for Calradia food would be scarce and a LOT of people would be poaching, it'd make a lot of sense that a lot of battanian peasants would know how to hunt, on top of owning a bow to hunt with, of course.
Speaking more broadly, I think every faction in general should be more lightly armed and armored (especially armored.) Perhaps adding some diversity to the leather (cuir bouilli for example) armor selection and some heavier clothing would help alleviate that issue, alongside weaponry that would better suit the time period the game is built on. More diversity in the peasants' clothing would be nice, they all look a little too prim and proper when they're just mere recruits from a pre-medieval society. Also, the game should have wildly different mechanics for each factions when it comes to battling enemies, managing logistics and handling intrigue and politics. Khuzait should be able to automatically "siphon" some food from the lands they're trying to take, having so much fast cavalry and generally employ horde tactics when in superior number while harassing and skirmishing before retreating when the battle is hopelessly against them. The Empire should have lower morale when fighting another Empire-based faction, they could also have unique formation (that really goes for all factions though) and a deeper intrigue system.
TL;DR=More diversity for light armor, less heavy armor, bows for Battania, more fragile HA, less advanced weaponry, more interesting game mechanics which could give the factions more flavor. Also, sorry about the post being messy, it's really late here.