Just a few thoughts:
1) There are often many different personalities within the exact same specialty. Or at least there are often going to be enough exceptions to the rule that it makes it hard to have a set rule in the first place (e.g. neurosurgery might be considered to have less laidback personalities, but I know of very laidback neurosurgeons).
2) It also depends on what you mean by "laidback". For some people, a "laidback" specialty is a specialty where they can work hard but then go home and not have long term care for their patients. For others, a "laidback" specialty has no patients at all, or very little patient contact. For still others, a "laidback" specialty is a specialty where they can work with people they get along with really well. For yet others, a "laidback" specialty is a specialty where they are their own boss and have a more controllable schedule. Etc. So it really depends what you think is "laidback" for you.
3) All that said, I wouldn't necessarily pick a specialty just because it has laidback personalities, and I wouldn't necessarily not pick a specialty just because it has intense personalities. In general I'd prefer to pick a specialty out of interest for the field. A possible exception is if certain personalities drastically affect you and your ability to perform well in the specialty, then I'd take that into consideration.
4) My personal answer is anesthesia and EM. They tend to be the most "laidback" for me, in my opinion. But they are absolutely intense when they need to be, such as when a patient is crashing, which is precisely when they should be intense and not laidback. But in general, they seem to be the most laidback to me. If you asked me "friendliest" in a more outgoing way, I'd probably say pediatricians. If you asked me "friendliest" in a more reflective manner, I'd probably say oncologists. Etc.