ISTP - one of my best friends in the world is ISTP and whenever I am stressed or upset or depressed, she is the only person in the world I want to hang out with. She just sticks me on her couch with a cup of tea and then goes off to do her own thing, making sure she is around if needed and that I am always supplied with more tea. She is just ok with everything. It's all good. She makes me feel I'm not isolated, but she doesn't crowd my space. When I'm ready to talk, she's ready to talk. She won't initiate. She's calm, relaxed, practical - the perfect antidote for my hot INFJ brain.
INFP and INTJ calm me to an extent, but the INFP is too emotionally invested for me to be able to completely let go, and although the INTJ is wonderful at first as they are so cool and logical, like cold water on my raging fire, they tend to try and fix things a bit too much. Problem? I'll fix it. That's not relaxing, though, feeling like something needs to be fixed.
ETA: Bless you, heart, for saying INFJ. But being one myself I can't help but think "how on earth can an INFJ have any kind of calming influence?" But maybe it's that cool, earth crust exterior (to use Ubol's metaphor from another thread). I suppose I'm too closely acquainted with the lava underneath.