Actually this may come as a shock to you all but Mega Aggron nor Shuckle has the highest possible Defensive Stat.
Although they are tied for having the highest Defensive Stat out of every Pokémon they do not have the highest possible Defensive Stat, that my friend belongs to Onix.
Despite Onix's other Stats being appalling it's Defense is fantastic at 160, giving Onix a maximum Defense Stat of 460 with 31 IV's, a Defensive Nature, and 252 EV's invested in it's Defense. When you multiply this by 6 Stages (4) you get:
460 x 4 = 1840.
Then if you gave Onix an Eviolite to hold it would increase it's Defense by 50%. So 1840 x 1.5 = 2760.
As you can see the Defense is far more than a 6 Stage Shuckle or Mega Aggron in Defense.
Shuckle/Mega Aggron = 230 Defense
With 31 IV's, a Defensive Nature, and 252 EV's invested in their Defense that brings their total to 614.
When you multiply that by 6 Stages (4) you get:
614 x 4 = 2456.
Thus Onix reigns supreme :D!