Agree with
The Quasar and
Idiotesque , as Roselia is undoubtably one of the most solid Special Defensive walls, Status Absorbers, Pivots, Spikers, in the metagame. It simply is versatile, and nonetheless effective. The standard Specially Defensive set can switch into just about any Special attack, and set up spikes. Hell. It has a 32%~ chance to live a Specs Fire Blast off of Charizard. In my opinion, that is a testament to it's bulk, as even pokemon which resist it risk getting killed switching into such an attack. On top of that, the offensive pivot set is a major threat, pivoting in and out of status, while spamming Sleep Powder and a surprisingly strong Sludge Bomb. Overall, I definitely agree with the nomination for Roselia Low-A / Mid-A.
I also wish to nominate Probopass for
Low-A. It is an extremely versatile Utlity Mon, Wall, and Volturner, being the only stealth rocker with access to Volt Switch in NU (Outside of the completely irrelavant Stunfisk and Nosepass). This great ability allows it to switch into things like Swellow, and with a Volt Switch slower than their U-Turn, bring momentum to your side. It is also an extremely good stealth rocker, being able to tank hits from large parts of the tier, and getting up rocks in their face. The Defensive set also acts as an extremely strong Speczard check which is a huge threat right now, being 3HKO'd, meaning it can hit back with a Power Gem / Gain Momentum with volt switch even after swtiching into a fire blast. Lastly, it's Magnet Pull set supports many of the current metagame's top sweepers extremely well, being able to trap something such as Klang or Metang, and opening giant holes for things like Jynx, one of the best pokemon in NU right now, to clean through.