I generally prefer to do legs and back in the morning. Stretching the back releases a lot of energy to help you wake up! Though I spend a lot of time on my legs, they’re still tighter than i’d like them to be. Also, because the muscles are so big, you can stretch deeply without going too far. It’s a really satisfying feeling.
I like stretching my hamstrings, lower back, and hip flexors. I have been running about 3-4 times a week for nearly 30 years and have completed 40+ marathons. As a result, these are the parts of my body that seem to need the most attention.