For TFT players unsure about which units to add for more strength, the candidates below are worth considering.
Yuumi
Regardless of the season in Teamfight Tactics, crowd control effects are always a crucial factor for players to secure victories. Successfully controlling the primary sources of damage or enemy tanks significantly increases the chances of winning by limiting the damage your team takes. It's no surprise that a champion with wide-area crowd control abilities, like Yuumi, is highly favored right from the 1-star level.

In addition to her crowd control abilities, Yuumi's shielding power when attached to an ally is incredibly potent in both offensive and defensive aspects. Paired with high-attack-speed carries, they assist Yuumi in charging her abilities much faster. Moreover, her shielding prowess ensures that her attached unit rarely experiences a sudden burst of damage from Assassins. With numerous benefits, Yuumi consistently brings immense strength to your team composition.
Tahm Kench
Roughly speaking, 90% of Tahm Kench's strength doesn't come from tribal synergy but rather from his 'Devour' ability. While some health or gold from Syphoner or Nomad traits is good, it can't compare to the limitless power Tahm Kench gains from devouring the right unit. With strategic feeding and appropriate stats, Tahm Kench can possess incredible resilience without relying on specific tribal synergies.

Furthermore, when Tahm Kench uses 'Devour' and eliminates an opponent in Teamfight Tactics, this champion not only brings a substantial amount of gold or valuable items but also enhances your chances of owning and upgrading essential units. The additional gold combined with the items becomes a valuable resource towards the end of the match, reaching the item limit. Therefore, if you need a front-line unit with excellent durability, strong damage, and decent 'gold hacking' capability, Tahm Kench is the best choice.
Jayce
Despite being designed as a flexible unit, Jayce undeniably excels much more in melee combat than at range. This is because Jayce in melee form possesses a well-rounded balance of defensive and offensive capabilities, a feature lacking in his ranged form. With skill effects reducing armor/magic resist by up to 50% at 1 and 2 stars, Jayce in melee form can amplify the team's damage to an immensely high level.

Moreover, this unit is not selective about equipment and can effectively wield a full set of defensive items while still dealing incredibly deceptive damage in melee form. Hence, in nearly every match, utilizing Jayce in the front line provides a sturdy and high-damage chess piece. Remember that in the late game, where opponents likely have fully equipped champions, Jayce's armor/magic resist reduction becomes immensely valuable for sealing the match.