Magneton
-Eviolite tank
-not a legendary
-access to a range of offensive moves
-obtainable in X&Y
-evolves thrice
-very viable competitively, especially with Magnet Pull
Magneton @ Eviolite
Ability: Magnet Pull
EVs: 252 Sp. Atk, 100 Speed, 156 HP
Nature: Modest
-Thunderbolt
-HP Fire
-Volt Switch
-Charge Beam/ Substitute
It has better defences than its evolved form if it's holding Eviolite, better speed and a base sp. atk of just 10 less than Magnezone. It does forfeit Leftovers recovery, but is a great option as a steel trapper. Volt Switch is needed to escape from any Heatrans that try to trap you.
Tyranitar
-Sp. Def tank, boosted by Sand Stream (reaches 492 in Sand with max sp. def EV investment)
-not a legendary; is pseudo legendary
-access to a wide range of offensive moves
-obtainable in Y
-evolves thrice (four times counting its mega evolution)
-very popular & viable competitively
Tyranitar @ Leftovers
Ability: Sand Stream
EVs: 252 HP, 76 Atk, 180 Sp. Def
Nature: Sassy
-Crunch
-Pursuit
-Low Kick
-Stealth Rock/ Fire Blast/ Stone Edge
OR
Tyranitar @ Assault Vest
Ability: Sand Stream
EVs: 252 HP, 76 Atk, 180 Sp. Def
Nature: Sassy/ Careful
-Crunch
-Low Kick/ Super Power
-Stone Edge
-Earthquake
Chople Berry was more viable before the introduction of Assault Vest, but in Gen VI I think Assault Vest outclasses the berry due to it boosting sp. def all round instead of just weakening one move.
I personally feel as if T-Tar has better items to use than its mega-stone, notably Lefties & the Vest, so I don't suggest Tyranitarite. Weakness Policy is another option, but Tyranitar is rather slow, so you're either going to have to pair it with D-Dance or Rock Polish.