Well, first of all, the three pokemon are all balanced in numbers, practically.
Should you care about attack, Terrakion's the way to go, especially with a STAB Stone Edge or Close Combat. Close Combat's generally seen, as Terrakion's defenses are the worst of the three, but balanced nonetheless. In my opinion, this falls second.
Virizion is the SP DEF monster of the Trio and should treated as such. Since it will be sought after by fire and flying attacks, I find this pretty much useless, as there seems to be more physical fire/flying attacks than Sp defense, but the defensive typing is good.
The best in my opinion is Cobalion, first of all because of its pokedex entry in White, stating that it makes "even unruly pokemon obey it". Also, it's the Base Defense monster, and being hit with Fighting moves and Fire moves, this is going to help a lot. Plus, it has a x4 resistance to stealth rocks and is immune to Toxic Spikes. While the movepool may be shallow, it should never be counted out from the trio.