Tablemon, ptcgradio, The Top Cut, Derium's Competitive Pokemon, The Pokemon Evolutionaries, pokeaim, The Flaming Spade.
TFS probably got me interested in competitive Pokemon in the first place, and pokeaimMD is clearly one of the go-to Poketubers out there. The others I've listed are TCG-related content, some competitive, others are pack-openings.
False Swipe Gaming is more recent, but really good, especially if you’re interested in competitive mons. The videos are always interesting and fun to watch. They also release on a weekly scedule, which imo, is the perfect balance of quality versus quantity. Def worth checking out if you haven’t already