There are a lot of unique realms in Battlemaster, though more of them go unnoticed than they should do.
Firstly I would mention Thulsoma, though it seems most don't actually know the culture, judging by conversations I've had, when they mostly assume the realm has a Saxon culture, which, ironically it is far removed from. Thulsoma is more of a mish mash of concepts, the devout religious extremism, the complicated belief in honour, combat, loyalty, the Anglo Saxon virtues, it even has quite a few Asian influences and cultural ideals. I still have much of the stuff to put up on the wiki, largely in regards to vast annals of roleplays I have saved on my laptop, all the Saxon Stormland/Anglo Saxon Chronicle stuff, and a lot more of the religious stuff.
Secondly, I would have to mention Averoth, which I notice hasn't been mentioned yet, they have a very interesting realm concept as being a Free State, and what truly makes them a Free State, and what they expect of their nobility, and their stance on things like religion, war, trade and government systems such as monarchies. Averoth is very underrated for this, which is somewhat understandable as its not been a major player for its entire existence, and now it is set to get far more interesting, there is a massive culture clash between the Saxons, and the Averothian's, as we disagree entirely on concepts of religion, honour, virtue, war, trade, soldiering, loyalty, just about everything really. Sextus and Haruka are a strange "couple" as well, they contrast each other well (Sextus being another unique character, that is really under represented) but both absolute rulers of their own Strongholds and realms, with a great deal of loyalty from their followers. Averoth and Thulsoma were confederated, and I always toyed with the idea of Haruka and Sextus doing a pseudo sort of King/Queen thing, with opposing strongholds on either side of the Stormland's. Me and Sextus, did have a wide variety of hilarious and fun Roleplays planned, though most we never actually did.
For a third realm, I have to mention Norland, it may not have been the most unique (As many say its just a copy of Makar) but the realm has always had a vibrant player base and roleplays, I've been in the realm since 2004, till it was destroyed, and it always had some manner of hilarious Roleplays, and it shall always go down as one of my all time favourite realms.
Some people have mentioned Arcachon, and I would agree, but I feel Arcachon has become much more of a "regular" realm, than one with a distinct culture and realm concept, particularly of recent years we have slowly gravitated towards the common sort of realm you find in the game. That said, we have a very well developed religion and Clan culture. Some people also mentioned Nothoi I think, well the realm is some what unique, but we really have not got much in the way of culture or Roleplay sorted. That said, it's quite amusing that Nothoi is the ancient Greek plural for Bastard, and our ruler is called the Diadochi, which means successor.