In League of Legends, there are some champions that are inherently very powerful, yet for various reasons, they are not favored by many players. As a result, these champions are played less frequently, their potential strength remains untapped, and they are not highly regarded by the community.
Below is a list of those champions that are “thought to be weak but are actually surprisingly strong”:
Teemo – Top Lane

Considered one of the most hated champions in League of Legends, Teemo is often not taken seriously by many players, with some even treating the choice of Teemo as an act of trolling. But seriously, Teemo can outplay many fighters in the top lane.
Especially during the laning phase, melee fighters who frequently use Conqueror often find themselves struggling against Teemo, who can zone them to the point of being “unable to breathe.” His Blinding Dart (Q) can poke and mitigate a large portion of the opponent’s damage. With this poking playstyle, Teemo keeps his opponents’ health low, making it difficult for them to last hit under the tower due to constant blindness.

Even in the later stages, Teemo can shine with his ultimate, Noxious Trap, making enemies very hesitant to engage in fights in areas without Control Wards. Therefore, major objectives like Dragon, Rift Herald, and Baron are often “mushroomed” meticulously by Teemo, ensuring his team can contest these objectives successfully.
Xin Zhao – Jungle

Regarding Xin Zhao’s power, he has relatively decent jungle clear speed in the current meta. When ganking, Xin Zhao’s strength becomes evident with the following rune page:

With the rune choices of Sudden Impact, Nimbus Cloak, Celerity, and Waterwalking, Xin Zhao can charge straight at enemies with Audacious Charge (E) + Three Talon Strike (Q) and relentlessly attack targets. This is because Xin Zhao receives approximately a 50% to 70% boost from the runes, and enemies will be slowed by the Red Buff and his skills, making it easy for Xin Zhao to hit and run effectively. With such chase potential, combined with the true damage from Challenging Smite, it’s tough for any prey to escape unscathed from Xin Zhao’s spear.
Syndra – Bot Lane

Unlike when playing solo lane, Syndra in the bot lane is much more dangerous. In lane, not only does her Dark Sphere (Q) have the potential to stun, but the support player will also help Syndra corner the opponents, creating significant pressure for enemies wishing to farm minions.
Some suitable support champions to pair with Syndra include Nautilus, Leona, and Blitzcrank—providing both tankiness and crowd control to help Syndra deal damage to enemies.
By the end of the laning phase, Syndra will become an extremely powerful carry for the team, contributing significantly in team fights. It is essential to ensure a balanced damage composition, as Syndra deals magic damage and will need a physical damage source from the top side of the map to prevent the enemy team from building full magic resistance.

You might consider buying Hextech Gunblade—10 less ability power than Luden’s Echo but 400 gold cheaper, plus it can slow enemies to increase the chances of stunning them with the QE or EQ combo.