I voted for Battania. This was for two reasons.
First, Battania is fighting for it's own survival, not conquest. I like the idea of being part of a resurgent Battanian nation.
Second, it ticks all my boxes for my preferred playstyle. Aggressive infantry, some medium cavalry, and excellent archers. It's a combination that doesn't really exist outside Bannerlord. Sure, Vaegirs can function to an extent, but I've found their infantry to be unreliable at best. Then again, I'm primarily a Rhodok player, so perhaps I'm doing something wrong. I also really like the Battanian aesthetic.
All that being said. I look forward to being introduced to all these factions ingame, and may change by allegiance depending on who seems most worthy to me.