I'd say that Mudbray, and in turn, Mudsdale deserve S rank. While I haven't had multiple runs through the game, my first and only one was with Mudsdale, and it's a beast. Not only can you get it fairly early on in the game, as early as Route 4, but its stats are great throughout the entire game, Stamina is awesome for longer battles, and its movepool, while bad in a competitive scene, is all it needs ingame. Bulldoze + Rock Tomb 2HKOs most Pokemon early on and later when you get Haigh Horsepower and Heavy Slam, it can take on the later totems very well, and with its great bulk + Stamina, it can really take plenty of hits to make up for its lackluster speed. Evolving at level 30 is also earlier than any starter and the stat increase is giant, letting you have a very strong and bulky monster as you are still going against lower to mid level Pokemon when this thing reaches level 30. It has a great type combination with the starters, with Decidueye being the one I chose. Decidueye takes out Grass and Water types, Incineroar takes out Ice and Grass types, and Primarina can tank Water and Ice moves, plus with Mudsdale, you have a strong tank to take on most of their weaknesses, like Dark and Fire for Decidueye, Ground, Rock and Fighting for Incineroar, and Electric and Poison for Primarina. Yeah, those bring you bigger weaknesses to things like Ice, Water and and Grass, but those aren't really hard to find resistances for.
In the Elite Four, this thing is a life saver, for Hala, it can take on Primeape, Bewear and Hariyama(with potions), besides Poliwrath and Crabominable, which you can beat with either Decidueye/Primarina or a simple strong attacker. For Olivia, all you need is something to kill Relicanth and then you just solo the rest of her team due to her having 0 super effective coverage and Stamina boosts throughout the battle. Acerola is probably the most difficult E4 member to face with Mudsdale, but it certainly is possible, as you take on Sableye, Drifblim and Palossand while leaving Froslass and Dhelmise to teammates. Kahili, despite the type disadvantage, can take her on pretty well, with Skarmory being a pain but the rest of them just being Mudsdale bait, at least with Rock Tomb/Heavy Slam. Finally, Mudsdale can take on 4 out of 6 members on Kukui's team, depending on which starter you picked, since it walls Lycanroc, takes on Braviary, walls Magnezone, takes on Snorlax with Superpower, and if you picked Rowlet, also Incineroar.
Maybe it's just because I just finished up and still have the contributions of what Mudsdale did on my team in my mind, but i definitely think Mudbray deserves
High rank.