Does any one know, which is the best posture to be assumed for practicing meditation?
Bhikkhus, the bhikkhu following the practice of my Teaching, having gone to the forest, or to the foot of a tree or to an empty, solitary place, sits down cross-legged, keeping his body erect, and sets up mindfulness, orienting it.
Then with entire mindfulness, he breathes in and with entire mindfulness he breathes out.
The above is the standard posture for meditation (i.e.: sit down cross-legged). Even though I can't remain in this posture for a long time due to back pain, I personally believe this is the best posture for meditation. Having said that there are many other postures suitable, depend on your situation (walking, sitting, etc.).
There isn't a 'best posture' for meditation (although I have a yoga book which attributes all sorts of special mystical powers to the full lotus).
The best posture is the one that's best for you, that gives you a stable base (knees and buttocks) and lets you maintain your spine in a good upright position. For some that will be full lotus, others half or quarter, others burmese (all on a cushion), and for others it will be seiza on a cushion or bench.
You definitely shouldn't be deciding how to sit based on answers from people on the internet, you should go to a temple or monastery where experienced meditators with a variety of cushions and benches can observe you and help you find the posture that's best for you.