this list is completely based on manga and contains spoilers
Breath of The Beast/ Love Pillar They both deserve to be in the list as they are a deadest match in terms of pure power.
The Insect Pillar, She lacked the strength of cut off a demon's head. But creating a poison that is strong enough to take down an Upper Moon is not everyone's cup of tea.
Breath of Thunder, At starting he was only a threat when he passes out. But later on he displays a monstrous level of raw power as while as increasing his already unmatched speed which took down the Upper Moon 6.
The Sound Pillar, He was able to hold off an Upper Moon while being poisoned. Also, his weapons make him a really versatile pillar.
Dance of the Fire God/ Water/ Thunder, His dance of fire closely resembles the sun breathing and this guy can even jump above a ranking or two if he can control his marked state on command.
The Water Pillar, He is the one who outshines everyone with his combat skills, as every fight is a cakewalk for him. In his marked state, he can even go 1v1 against Akaza.
the Mist Pillar, Trust me this guy is a monster and can easily be on #1 if had a plot armor. He was the youngest pillar in history and mastered his breathing at only 14.
Flame Pillar, This guy held off Akaza in a 1v1 state simultaneously protecting Tanjiro, Inosuke, Zenitsu and all other 200 passengers of the train. He nearly chopped off the head of Akaza without even being in a marked state. Guess, We'll never know his full potential.
Honestly , this guy is a beast. Upper Moon 1 stated that he had reached peak physical condition for a human similar to Giyomei. He was able to fight on par with Giyomei while fighting Koku. There isn't much to say about his strength as he was recognized by Koku the strongest demon moon.
Rock Pillar, Tanjiro and Inosuke confirmed him to be the strongest in the Pillar training arc. Even the upper moon Kokushibo, stated that he is the most skilled demon slayer he fought in hundreds of years. He also went 1v1 against him despite being blind.