I noticed Grimer, and by extension, Muk, have the unfortunate task of being on the D Tier. Whilst I don't exactly agree with that, I'm probably biased because of my mono poison run where my Muk is putting in quite a lot of work. However, it most assuredly does not belong on B, so where does that leave us? Solid C, and here's why.
Grimer - Muk for C Rank(As opposed to D where it lies now)
Availability: No badges! Just get yourself the Old Rod and it's time to go.
Movepool: Quite good, actually. Notable moves include Gunk Shot, Curse, Shadow Sneak and Punch, Sludge Bomb AND Wave, Flamethrower, Brick Break, ect ect.
Muk and Grimer are an interesting pair of sludge blobs. The polar opposite of Weezing, they have good attack, HP, and special defense. It has 2 okay enough abilities in Stench(a nifty flinch chance if you manage to go first) and Poison Touch, which turns any of your contact moves into a 30% chance to Poison the target. It's movepool is also VERY good, and with catching a wild Duskull, if you give it one level(from 32ish)you have yourself 3 egg moves right off the bat: Shadow Punch and Sneak, as well as Curse. Curse is the main bread and butter of what Muk and Grimer are going to be doing. I've noticed that right around Shade, a lot of major boss battles(Shade himself, Cal, Kiki, Victoria)are all dealt with incredibly well with a Cursed up Muk. After 2 or 3 Curses, it's nearly untouchable and given that speed isn't an issue because you have Shadow Sneak, Muk turns into a devastating sweeper with minimal effort. That's not to say Grimer and Muk don't have their issues.
The biggest issue is how long it takes for Grimer to evolve. Level 38 is not a low level, and trust me from experience, you NEED the added bulk evolution gives you to be viable here. Other than that, Grimer/Muk's natural level up pool is...quite mediocre, to be frank. Sludge Wave/Bomb and Gunk Shot are your best moves, and whilst they're great, the first two go off of your worse Sp.Atk stat, and Gunk Shot not only has iffy accuracy, but only 5 PP. Aside from that, there are just some battles where you won't get a chance to set up, or the enemy is just packing a lot of coverage in Psychic moves(a certain Roserade and Lucario jump to mind). Overall though, the sludge blobs are a very solid pair of Pokemon, but not without some pretty glaring flaws that can hold them back. If you want a surprisingly strong Poison type, give them a try, you won't regret it.
Tl;dr: Specially bulky and physically strong, but slow and needs a bit of work to make use of said talents. Makes Shade's gym an absolute joke with Curse/Shadow Sneak, incredibly helpful against Noel's fat pink blob of a Clefable, overall solid sweeper in the right scenario. Has some glaring downsides in typing, natural level up movepool, abysmal speed, okay defense.
EDIT: Realized I said it didn't belong in C, yet put it as "solid C" this has been corrected and I am in my shame corner.