If you mean defensive as in physically defensive, Cloyster would win with base 180 Defense.
Specially Defensive it would be Regice with base 200 special defense.
Mixed wall-wise I'd probably back regice... it has defenses of 80/100/200 (HP, Def, S.Def) which are really solid, though Pokemon like Lapras (130/80/95) and Mew (100/100/100) are all pretty solid options too.
Frost Breath
