Regular Yoga.
Don't go to extremes with working on flexibility, injuring yourself and having to heal will waste you more time than regularly doing Yoga.
Yoga asanas? Although I do not see why someone would need to do spiritual working specifically for flexibility when that is directly solved by asanas. Suppose there are underlying health conditions that hinder one in one way or another that would be another case.
if u like yoga u can trying tantra yoga or asura yoga(for asura yoga better ask gods becoz many corrupt version my suggest ask with baal or belial or vepar,or satanachia or sitri ).