At first I was flamed at for saying this, and at the time I understood why. I made the claim that Chesnaught belonged in C Rank because its typing was god awful. People were keen on letting me know that I was wrong, and to be fair, I was a bit harsh. Chesnaught isn't nearly as bad as I once thought, but now after studying a bit more and using him in OU, I can safely say that I still don't agree with Chesnaught in B rank. So, I'm still probably going to get told off, but this time I've decided to explain myself more thoroughly.
The main argument used to defend him is the fact that he resists EdgeQuake, but this isn't a valid argument in my opinion, as both types are basically limited to physical attacks, which won't be used against Chesnaught unless the opponent is inexperienced or it hits him on the switch in. I know of another Pokemon that resists EdgeQuake, but because of its typing was limited to NU last gen: Torterra. Now in no way am I saying that Chesnaught is as bad is Torterra, far from it. But it's easy to see why I made the comparison.
It's obvious that the main thing that makes Chesnaught viable is his excellent hidden ability: Bulletproof (Funny how many 6th gen Pokes are only used for their HA). With Bulletproof, his excellent base defense stat of 122, and his signature move Spiky Shield all come together to give him a niche in OU. I have no complaints with these things but I don't think they make him quite as useful as his fans make him out to be. I still stand by my statement that Grass / Fighting sucks defensively. Ice, Fire, Flying, Fairy, are very common offensive moves, three of which are primarily special attacking types. Chesnaught cannot take special attacks well at all, and you could argue that he doesn't need to as he is a physical attacker, but just look at Ferrothorn and Skarmory (yes, I KNOW they are higher ranked I'm just explaining something). Both due to fantastic typing can sponge many special attacks well even with no investment. If that isn't a fair comparison, then how about Suicune? Suicune lacks reliable recovery, but gets access to Calm Mind, which boosts its uninvested Sp.Def stat to make him amazingly bulky, as well has giving him the ability to hit back very hard with Scald or Ice Beam. Chesnaught's base 100 Attack is nothing to laugh at, but it is almost never fully invested in due to the fact that he relies on his defensive bulk. Not only this, but he also has a very predictable set. The set I personally run is in my opinion, his best set: Leech Seed / Spiky Shield / Synthesis / Hammer Arm, and this is the set I almost always see. Alternatively, you could run Earthquake in place of Hammer Arm (you're either checked by Ghost or Flying, so pick your poison). You never see offensive sets for Chesnaught, and for good reason: they're very bad. Bulk Up can make Chesnaught's Attack very high, but it does nothing for his poor Special Defense, and Talonflame is still going to be a problem. Belly Drum is laughable on a Pokemon this slow. Plus, special attackers such as Mega Charizard Y, Keldeo, Greninja, Volcarona, Alakazam, Sylveon, Latios, and Thundurus are huge threats for the Spiny Armor Pokemon.
Disagree with me or not:
B ===>
B- Rank.
(PS: This is my last discussion on the matter, as I already mentioned this before and I don't want to rub anyone the wrong way.)