You're forgetting about crustle who has spikes AND rocks and can shell smash (lol who even uses shell smash on it though). Crustle is still just as good on offense as garbodor is on balance and having both rocks and spikes is what makes it stand out. Another really good spikes stacker is qwilfish, it has some good abilities and great speed for a stacker meaning it can taunt all other non-mental herb spikes stackers in the tier before they set up, bar accelgor. Qwilfish is also quite a versatile pokemon, all of its abilities are useful and can play multiple niches for your team allowing you to run multiple sets to trick your opponent. Qwilfish also has a great movepool and works a lot better on hyper offense than garbodor does. Seriously though, this thing is more versatile than people give it credit for. It can be an SD set, lead with explosion or destiny bond, Defensive with T-wave and scald or even a swift swim rain check since it outspeeds kabutops and can ohko ludicolo (don't quote me on that) with a jab to the dome.In my opinion, Garbodar---->A-
Reasoning: Garbodar is far and above the best toxic spikes and spikes stacker in the entire tier, besides accelgor, who is OK at spikes, and Ferroseed, which is not as fast as Garbodar, and doesn't get toxic spikes. Not just having great typing in mono-poison, having stab gunk shot, and also access to black sludge which can nullify trick/switcheroo, unlike ferroseed, but it also gets a great ability in aftermath and not as bad bulk as accelgor at 80/82/82. For that reason, I thin Garbodar should be promoted to A-.
EDIT: You guys also might have to think about Pangoro, because of Knock Off, Drain Punch, and Gunk Shot from the ORAS tutors. But that's speculation. lol that sounded like I was selling something
Moral of the story: Garbodor is not the best spikes stacker at all. All the stackers of the tier play very different roles and thus there really can't be the best one. Garb is better on balance/bulky offense while qwil is better on hyper offense and fills more of a niche than other stackers.