What are the best exercises to increase flexibility?

Stretching, whether static or dynamic is only one component of improving flexibility/mobility. Trigger points (aka muscle knots) are also an important factor in soft tissue flexibility. Stretching isn’t very good at removing trigger points so you need to do some sort of trigger point/myofascial release work to get rid of those.

also, frequency is more important than duration. I’d rather have someone do 10-15 min. of flexibility work every day than only one yoga class every week. If you have particularly tight areas, doing some work on these spots (even a couple of minutes) multiple times per day will get you the best results.