Well, I've only been in 1 regiment since I started playing BG2 back in 2008. So I can only say The Royal Recruits. A few members can be a bit harsh at times, but once you understand the dynamics of the clan people usually stay for the long run. Like all the officers in The Royal Recruits have been in the clan for a minimum of 5-9 years. Usually nowadays, once a serious recruit joins, they usually hang around for a few years, so most of us are rather close. People probably hate the fact that we only promote if a spot opens up, so we usually have a few unhappy privates begging for promotions
(The average wait time for a promotion within The Royal Recruits is about 1-2 years!! AA, though the fastest since I've been around was a member named Alpha back in BG2, I believe he got promoted from Private all the way up to Sgt within 12 months.)
If I had to choose another regiment, 1er under Herlev seemed like a nice bunch of lads, since we knew him quite well back in BG2 under his old clan. The 17th also seems like a nice bunch of people!