Thanks to magumi-yashi for suggesting this Mega!

Upon Mega evolving, Registeel gets the ability Battle armor, which works very well for its tank role. It gets big boosts to its defensive stats and a boost to its attack, and like mega Regirock lowered speed. Its appearance changes to where it appears to have knight-like armor and holds a huge, powerful looking Lance made of steel. (Notice the trend I'm doing with the regi pokemon?)
Stat boosts:
50 defense
50 sp defense
30 attack
-30 speed