As a combatant, he was probably low/mid Council level. Obviously well below the likes of Maul(even SW.com states that Maul "nearly killed" Qui-Gon during their first encounter on Tatooine.)
It was Qui-Gon's spiritual enlightenment and connection to the Living Force that ultimately made him one of the greatest Jedi of all time.
It always bothered me how underwhelming Qui Gon is combat wise compared to the top tier jedi and sith. How can you have such a strong connection to the force and even discover immortality yet be so average in combat?
“Average” is relative term. Qui Gon was still stronger than 99% of all Jedi who ever existed. Remember that most Jedi are around Jedi Knight level, like those we see in the background in episode 2. Qui Gon may seem underwhelming because often the story revolves around the strongest Jedi, that’s it
In Master & Apprentice, Qui-Gon was invited to join the Jedi Council. So he's literally Council-level. His performance against Maul is more impressive than most feats from the Jedi Council members throughout The Clone Wars.
Going down the list:
Qui-Gon is unquestionably above the likes of Jocasta Nu and Coleman Trebor. Kcaj, Poof, Alliee, Rancisis, Yaddle, Tinn, and Kolar have pretty much no feats in Canon.
Even Piell surrendered to a couple droideka, but Qui-Gon also chose to flee from them. So it's not a stretch of the imagination to assume Qui-Gon and Piell are on the same level.
Qui-Gon's showing against Maul is also more impressive than Depa Billaba and Adi Gallia's individual showings against Grievous. Eeth Koth is a little more impressive than Depa Billaba and Adi Gallia, with him holding out against Grievous while injured and also lasting for a time against pre-prime Vader. I could see Qui-Gon being roughly comparable to Koth.
Plo Koon, Ki-Adi-Mundi, and Shaak Ti all lack dueling feats in Canon, only really being depicted against blaster-wielding opponents, so it's impossible to gauge there.
This leaves Kit Fisto, Obi-Wan, Mace Windu, Yoda, and Anakin as the only Jedi Council members who I would rank above Qui-Gon. I'd definitely consider Qui-Gon as roughly equal to the likes of Eeth Koth and Even Piell.
By my estimate, this puts Qui-Gon in roughly the top 15 best Force users of his era, out of literally over 10,000. Possibly even in the top 10, depending on where you rate him relative to Asajj and Savage. That's by no means "average."