It's funny, in Warband, my least favorite faction -from an aesthetics standpoint, was Swadia (I liked their playstyle though). Their whole design just felt very generic and cliche, like the stereotypical Western European medieval kingdom you'd find in any pop culture medium.
Yet in Bannerlord, I find that Vlandia -the ancestors of Swadia (and the Rhodoks!), looks very interesting, with its troops having nice distinctive features, as if their troops actually belong to a unique culture. In addition to the units, I believe the one map in Skirmish Mode -the one with the stables, market stalls, and pennants stretching across the streets, is a Vlandian settlement. That map has a cool atmosphere, something I feel was missing from Swadian cities and villages in Warband. This goes to show what an excellent job Taleworlds has done in giving every faction a unique, intriguing look and vibe. Hats off to the devs.
A close second though is the Aserai. I'm absolutely loving their aesthetic right now. The dusty, arid settlements hugging a precious life-giving water source, and not to mention how fierce the Mamelukes look. In addition, have you guys checked out their Bedouin unit? Fastest horse in the game, plus if you take the (I think) improved armor perk, your character is wearing this leopard or cheetah skin draped across their shoulder. How cool is that?