You already know who it is.
Tokyo Ghoul by Ishida Sui ended almost 10 years ago, but fans still discuss who is the strongest character in the series. Almost every discussion includes Kaneki, Furuta, Arima and Juzou. They are always compared at their peak — post-dragon Kaneki, Arima is just Arima (he was always at his peak), Furuta during the Dragon arc, and Juzou with Arata Joker.
Let’s begin with Kaneki — One-Eyed King, Miracle Human, Dragon — it’s all about him. Kaneki has mastered his kagune and upgraded his kakuja in post-dragon form. His regeneration is on another level, speed, strength, endurance, pain tolerance, high skill in combat — all of this makes Ken one of the strongest ghouls in the entire series.
Arima Kishou, White Reaper of CCG, Half-human, is the best ghoul investigator. Calm and cold personality, stoic demeanor, mastered swordsmanship with the strongest quinque (SSS-rated Owl), half-human agility and physiology. He could defeat anybody, but he lost to Kaneki, even though they were equal and both of them had no intention of killing each other.
Furuta (Kichimura Washuu), Washuu King, Rank 1 ghoul investigator and artificial ghoul with Rize’s kagune (like Kaneki Ken). Half-human and ghoul physiology, Rinkaku kakuja and kagune, superhuman strength and regeneration, and mastery of swordsmanship — all of this makes Furuta almost equal to Kaneki, but he still lost.
Juzou Suzuya, CCG’s Jason, The Reaper, Dragon general. His physical abilities are on another level — he is equal to ghouls in the fight, that’s why he entered the duty of ghoul investigator without attending the academy. Superhuman speed, strength and agility, as well as swordsmanship, pain tolerance, and Arata Joker (a kakuja which is used as armor and increases the wearer’s abilities, but causes bleeding) make Juzou one of the strongest humans.
So, according to this information, fans consider Kaneki to be the strongest. The abilities of these characters have similarities — superhuman strength and so on, but Kaneki is on another level. Arima had an accelerated aging and glaucoma, that’s why he would still lose to Kaneki (even without his plan to see if Kaneki is strong enough to call him the One-Eyed King). Furuta is really dangerous and was almost equal to Ken in their last fight, but his ego made him lose. Juzou can be considered as the strongest human with his Joker, but he can’t wear it all the time, so he's no match for Kaneki either. Well, that makes the main character the strongest in the series — it was quite obvious, but still interesting to compare other powerful characters with him.