The most efficient agent for the absorption of ${\mathrm{SO}}_3$ is 

$98\mathrm{\%}{\mathrm{H}}_2{\mathrm{SO}}_4$  is used for absorbing dense fog of acid which is farmed by dissolving ${\mathrm{SO}}_3$  in water. Hence 98 % H₂SO₄ is the most efficient agent  for the absorption of  ${\mathrm{SO}}_3$