Well, obviously it'd depend on the rest of your team.
Type disadvantages need to be taken into account, and Virizion's x4 weakness to flying is crippling.
Virizion Clam Mind Giga Drain, along with Life Orb can do a lot of damage. Immunity to leech seed. However, that's probably its most reliable special move, since Focus Blast only has 70% accuracy. Still, you could give a set of Calm Mind, Giga Drain, HP Ice, Focus Blast, or a physical set of Swords Dance, Leaf Blade, Secret Sword/ Close Combat, Stone Edge. If you manage to set up Swords Dance/ Calm Mind, it will be able to deal with flying types, providing that it outspeeds them. But honestly, I was trying to get Virizion in game and used Acrobatics with an Emonga that was ~8 levels below it. Still KO'd. :/
Cobalion can have either a calm mind/ swords dance set too, and with x1/4 resistance against dark, Justified will raise its attack for less damage than Virizion will take. While I'd personally prefer a special set in favour of HP Ice's accuracy, Cobalion's special move pool isn't that appealing. Calm Mind, Focus Blast, HP Ice, and Flash Cannon is probably the only viable moveset. It has much more to choose from its physical move pool; Swords Dance, Sacred Sword, Stone Edge and X-Scissor/ Substitute. Then again, one STAB fighting move will cripple Cobalion, much like one flying move (without STAB) will cripple Virizion.
If you haven't got a steel type, go with Cobalion. Steel typing means immunity to Toxic, decent speed and high defence. Otherwise, Virizion can be nice to use but it has a lot of weaknesses, so it's difficult to pull it off.
