I'm going to contribute my two cents to this discussion about Cofagrigus, Roserade, Tornadus and Virizion.
As a player who's never successfully used Cofagrigus in a ranked match, I cannot say too much about its viability... only that when it does come up on the opponent team it rarely if ever gets to set up, because Togekiss the stallbreaker just doesn't let it have a free two turns, and Cofagrigus just can't beat it even under TR. It also can't beat Meloetta, given that it's played right, and honestly it requires a decent amount of team support to function - for example, you can't run it on fast HO simply because other members of the team won't appreciate Cofag dying while TR is up. A 2-turn set up requirement is also pretty bad because in tougher matches you probably won't find time to set up both TR and NP. Because of these reasons, I'd argue pretty strongly that Cofagrigus is an 'A' tier Pokemon. It's not S-tier simply because it's not a one-man-army like, say, Raikou is.
Roserade, on the other hand, defined the UU metagame for the entirety of BW1 and it's been a great attacker and supporter in BW2. It doesn't need team support - it IS the team support and it grabs so many free turns, either from scaring off bulky water and whatnot or spamming sleep powder whenever it comes in. TSpikes and Spikes are really useful, not to mention immunities to Toxic and Leech Seed. I think it's honestly a more deadly and versatile threat than Cofagrigus, so I'd nominate it to S-tier.
I also find Tornadus underrated, but A rank is fine for it, though it shouldn't ever drop below that.
Virizion, on the other hand, always struck me as the odd one out of the four musketeers, possessing a low attack and a sky-high special defense mitigated by that horrendous 4X weakness to flying. It doesn't run CM well, due to it being outsped by stuff like Heracross and Scarf Darmanitan and being forced to use Focus Miss as STAB; it also sucks at SD because of that inferior base attack, which should put Cobalion above Virizion because of its superior SD and CM sets. Cobalion also has much better defensive typing and has more utility as a tank. Thus, I'd say put Virizion into B-tier and Cobalion into Virizion's place.