I'd like to nominate Armaldo to go from B- to B+. While Armaldo is not a very well known Pokémon within the tier and its speed is not at the top of the tier by far, it makes up for it with its high attack and defense, giving it the capability to be run with a variety of different sets.
1. Assault Vest Armaldo. Armaldo's movepool provides many different opportunities for AVest sets, including the hazard-removing Rapid Spin, the Priority Aqua Jet, which when paired with Swift Swim can make a powerful combination. Its physical moves like Aqua Tail, Knock Off, Rock Slide/Stone Edge, Earthquake, Body Slam, Brick Break/Superpower, and X-Scissor provides the opportunity for a powerful yet bulky sweeper. This also accounts for Armaldo's low special defense stat. For example:
Armaldo (M) @ Assault Vest
Ability: Swift Swim
EVs: 252 Atk / 252 SpD / 4 Spe
Adamant Nature
- X-Scissor
- Rapid Spin
- Stone Edge
- Knock Off
This gives Armaldo a SpD stat of 388.5, or 389, which will tank many special water attacks from the tier's more dangerous water types such as SS Gorebyss or other special attackers that may use Ancient Power or Power Gem.
2. SetUp Armaldo. Armaldo's low speed can be paired up with a Trick Room for a powerful sweeper. With access to Swords Dance and its before mentioned physical movepool, Armaldo can be a physical tank that can take many different attacks. Again, you could also run this with Swift Swim on a rain team instead of Trick Room.
Armaldo @ Life Orb
Ability: Swift Swim
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- X-Scissor
- Swords Dance
- Stone Edge
- Knock Off
This gives Armaldo many different capabilities including coverage and the possibility for a high speed stat on a normally low-in-speed tank.
3. Choice Band Armaldo. While Armaldo's base attack at 125 ranks at the 8th highest in PU, running a banded set gives Armaldo even more strength, and YET AGAIN, can be used with a rain team or trick room.
Armaldo @ Choice Band
Ability: Swift Swim
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Superpower
- Knock Off
- Stone Edge
- Aqua Jet
Having Stone Edge for STAB, Superpower for coverage and power, Knock Off for destroying possible sets like White Herb/ Shell Smash, and Aqua Jet for priority and possible combination with rain makes Armaldo a threat within the tier for any Pokemon within the tier, even for some of the highest defensive beasts in the tier such as Avalugg, Bastiodon, and Carbink.
4. Support Armaldo. Armaldo's access to common moves such as Stealth Rock and Toxic, combined with its high Defense and HP, make it an ideal member of a team for setup.
Armaldo @ Leftovers
Ability: Battle Armor
EVs: 252 HP / 4 Atk / 252 Def
Impish Nature
- Stone Edge
- Toxic
- Rapid Spin
- Stealth Rock
This set provides Armaldo a boost to its already bulky frame and also provides the opportunity to take out possible sweepers with Toxic, and to set up Rocks for dangerous mons in the tier like Glaceon, Articuno, Flareon, Regice, and Rotom Fan/Freeze, which are all affected by the factored rock weakness.
Overall, these sets can make Armaldo just a generally bulky yet physical member of any team in the PU tier. It is more often than not overlooked, but it has the capability to bolster any team's defensive or offensive presence.