My recommendation:
Silnaria (Atamara): I play the Monarch of Silnaria, so while I am a bit biased, I am also in a position to clearly state what I believe are my realm's strengths for a player seeking a new realm. We are the most recently founded realm on Atamara, and were essentially created (through secession) by the process of eliminating those in our realm who stood in the way of progress and refused to help create an environment which cultivated new nobles positively. As such, I can state with certainty that every single lord of my realm is not only actively engaging for a new knight to interact with, but are also outspoken about what they believe in. We are a spirited group, we have many strong and unique personalities and are in the process of creating our own culture from many diverse backgrounds. We are THE center of Atamara, and will thus always be in the middle of whatever action is going on. My character, the ruler of Silnaria, is one of the most controversial figures on Atamara and has those who both despise him and those who love him. You won't get bored here, of that much at least I can assure you.
A side note: While I know many people WILL oversell their realms to gain extra nobles, I welcome any direct questions of my realm, my playstyle, etc... through private messages if you want to see if what I have said holds up. I encourage any interested in Silnaria to ask any other player who is there, and they can back up everything I have said. I believe Silnaria hosts the most active realm per total noble base on Atamara.