I hear Riombara has a promotion going: The next nine nobles that join get a free region command, as fast as they can run an election! ;p
In all fariness, this should go for most realms since 4/10 realms have more regions than nobles, and even Thalmarkin with one of the highest ratio nobles/region we have currently (and quite constantly) 4 regions without lords.
But yeah, BT definintely seems like the continent to join, whichever realm it is! Wars in all realms except one, and tons of chances for promotions all around. And soon there will be a whole "new" area needing population! :p