I don't know about all time but the greatest Test performance of this century was by VVS Laxman in 2001. Australia had crushed Pakistan in the 1999 World Cup plus they had won 16 Tests in a row. India had lost the previous match and they were staring at a humiliating defeat by an innings at home. That result would have also meant losing the series. The mighty Australian juggernaut was unstoppable!
Under such tremendous pressure and utterly hopeless conditions, Laxman scored an epic 281 runs which won them the match. India won the next match as well which meant that they won the series. This was all done against the greatest Test team of the era.