There is no posture like Siddha asana
"Of the different postures, eighty-four are the main ones. Of these, again, four, viz., simha, bhadra, padma, and siddha are said to be excellent. Of these too, it is only siddha, that is the most excellent. Thus the yoga texts declare." (from Ramana's "Self Enquiry")
When a Muslim visitor asked Bhagavan about asana, he replied that "abidance in God is the only true posture." (Talk 234)
On another occasion, he said "nididhyasana (one-pointedness of the mind) is the best." (Talk 557)
The one-pointed mind focussed by vichara (Self-enquiry) is indeed pure abidance in the Self.
When the ego rises up, confusion ensues and the world of differentiated objects is taken to be real, He goes off searching for the best asana. In truth, one is the Centre of all and forms the basis for all - the seat upon which all else abides, including the multitude of asanas. This is siddhāsana.