Knight
What makes Knights strong in the current meta is that this class provides excellent durability for the entire team against various types of damage. In a game of Teamfight Tactics, you will encounter many team compositions with diverse damage sources, so the ability to absorb damage from items may sometimes be insufficient. At that point, the direct damage blocking ability and its application to all forms of damage by Knights will be an incredibly powerful tool for players to survive.
Moreover, the strength of Knights lies in the fact that this race is extremely easy to play and useful at all stages of the match. By using Knights, you can start off very strongly with Leona, Poppy, or the annoying crowd control from Nautilus, Thresh. Meanwhile, expensive units like Darius, Garen, or Taric are always strong choices in the late game. This versatility has led to the Knight class appearing in most meta compositions like Requiem, Celestial, and Divine…
Sorcerer
With many reroll team compositions being nerfed, the pace of the game is slowing down, and the meta is shifting towards strong late-game team compositions. Therefore, units with powerful abilities and high maximum mana are becoming very popular in the current meta. This has led to the Sorcerer class (whether at 2 or 4 champions) being increasingly used. The reason is quite simple; you cannot have enough mana items for the entire team and must rely on Sorcerer effects to increase mana.
Besides the necessity of the Sorcerer effect, all units in this class are extremely strong, and players should invest in them. Syndra can be considered an irreplaceable piece of the Celestial team with the ability to throw Assassins or threats away from the carry. Karma and Teemo are already famous for their massive area damage. Ivern may not deal much damage but has excellent crowd control capabilities.
Mystic
Patch 11.12 can be seen as the return of strong magic units as Karma and Vel’Koz are becoming extremely popular. Not stopping there, the Requiem and Celestial compositions have champions that deal magic damage very well, so magic resistance is very much needed in the current meta. This makes the Mystic class an indispensable factor for your team to have enough durability and deal effective damage at this time.
Another point that makes Mystic extremely popular is that all units in this class are highly versatile. This greatly enhances the defensive/offensive capabilities of your team right when you bring them onto the battlefield. For instance, just with a 1-star Kindred, your carries can survive a long time in combat. If you want additional crowd control effects, units like Lulu, Ryze, and Morgana are very strong choices.