Just finished the 12th class of my mini-challenge, which I've been enjoying immensely. It's really been helping me to feel more calm during the end of semester zaniness (although the time commitment does sometimes feel like a struggle when I have piles of things on my desk). I was thinking this evening about my current favorite and struggle postures -- it makes sense that they are opposites of each other.
My favorite (if I had to pick one, though there are others I like too) at the moment is standing separate leg stretching pose (Dandayamana - Bibhaktapada - Paschimottanasana). I like all of the poses in which you extend the spine and reverse the direction of gravity. I probably like them because I'm reasonably flexible in this direction, so I can get fairly far into the posture. Once you can get your neck and spine in alignment upside down, it really feels great. This pose stretches your calves, hamstrings, and glutes, too, before you get to the spine -- and it's refreshing for the face & brain since the blood rushes forward. Not a pose to do without plenty of warmup, or without careful attention to the knees -- but by the time we get to it in the sequence, we've done a lot of standing poses and so the reverse direction feels great.
My least favorite pose, only two later in the series, has been my least favorite for a while: standing separate leg head to knee pose (Dandayamana - Bibhaktapada - Janushirasana). I've gotten better at it over time, but I still have to do the beginner version with the front knee bent. The front compression poses are all difficult for me (since I've got a lot of body weight to the front), and this one requires core balancing along with the compression. It's a powerful pose -- it will clear your sinuses when you didn't know you had congestion -- and it works on the internal organs as well as torso alignment. But I find it a struggle. Which means it's what I need to work on the most.