Trikonasana never fails to bring a sigh of contentment for me. It's not the flashiest pose, so my satisfaction with it isn't to do with accomplishing something difficult. It just always seems to be what my body needs to do. Could be because it often comes near the end of a standing sequence when I am dying to straighten my leg. What's your happy pose?

Mine is the plow pose. I’m pretty flexible, and the intense curl gives me a lovely stretch that just feels oh-so-good…!
Supine spinal twist…feels so good at the end of a yoga session. Sometimes I’ll just do it when my lower back is feeling tight.
butterfly on some days…
Urdhva hastasana
I love the stretch thru my whole body. I bend back as far as I can. By reaching my arms up and back, it relieves the stress during the day. I do this multiple times thru the day since I have a desk job.