Chesnaught is definitely at least a B, just because it's not seeing a lot of play right now doesn't mean it's not excellent. It's one of the best spike-setters and scouts in the game right now, its physical bulk is incredible, its grass typing paired with bulletproof allow it to wall and set up on Mega-Venusaur of all pokemon and switch into a lot of Aegislash/Gengar builds without fear. Plus its movepool is huge, it packs synthesis, leech seed and spiky shield for recovery; spikes and taunt to make it a decent lead and bulk up, swords dance, EQ, Hammer Arm, Wood Hammer, Dragon Claw, etc for coverage. Seriously this guy is a sleeper hit.
I have been using some Chesnaught before, the experience was rather nice, if you team is good enough against Talonflame and possibly some psychics than it definitely worth a try(I assume every team should have some sort of fire/ice solution).