I'd say P'Li has this hands down. Azula is equal to fighters like Zuko and Katara at the end of the series, but P'Li has only been defeated by multiple LoK masters and elites, it's not even conceivable for experienced strategists like Lin, Tonraq, and Kuvira to devise a strategy that included someone 1v1ing her.

Azula's greatest strengths are useless in this fight. Azula takes some time to fire lightning. Zuko had only redirected lightning once before and he was clearly capable of discerning when she was firing. P'Li's combustion bending not only fires faster but can bend in arcs and he used in such rapid succession that extremely agile combat masters like Kuvira, Lin, and Suyin cannot close distance. Her intense blue fire is great against most opponents, but P'Li can bend dragon's fire very quick and well even after being kept in torturous conditions for 13 years.

P'Li's greatest advantage only continues to be an advantage. She might very well near match Azula in pure firebending, but with her combustion bending, she easily takes this. She can keep several masters pinned down and every single effort at negating her combustion bending was done with multiple masters, all of them which were much, much faster than Azula. Azula got snuck up on and taken down in 2 seconds by Ty Lee, Lin fought multiple armed and armored Chi Blockers, even those attempting to ambush her.

Azula is a great tactician, but she needs tools to execute plans. Lin and Suyin both knew exactly how to beat P'Li in both the fights they had with her. However, both times included having master(s) distracting her while another one landed a finishing blow.

Azula is smart, but no one has been shown smart enough to 1v1 a combustion bender. Taking them always includes smart use of terrain (no terrain advantages as per rules) a distraction, and a well aimed boomerang, metal bent breastplate, or rock.