Best Team for Yanfei in Genshin Impact
Team #1: Yanfei Vaporize
Yanfei's main source of DPS is her charged attacks, which are furtherly boosted by the Vaporize reaction. Yanfei's needs are a Vaporize enabler (in other words, a Hydro character that can work off-field) and a shield, as she's pretty squishy, downside of being a Catalyst user.
Bennett's utility consists in providing a huge ATK boost to the team as well as Pyro resonance. He can't really be replaced as his contribute is unique, but other Pyro support characters will at least grant the Pyro resonance.
Zhongli's role is to prevent Yanfei's interruptions with his shield. Any other shielder does the job too, but Zhongli has the ability to keep his shield at 100% uptime and shred the enemies' resistances.
Team #2: Yanfei Overload
Yanfei's main source of DPS is her ability to trigger the Overload reaction, which does AoE Pyro DMG. This team and Overload in general is recommended only at early stages of the game, as transformative reactions perform better than amplifying ones (melt/vaporize) without proper late-game builds. Yanfei needs Electro characters to enable Overload and a shield, as she's pretty squishy, downside of being a Catalyst user.
Fischl comes with a considerable source of damage and energy generation, making her useful for both Yanfei and the other Electro teammate, which is generally energy hungry.
Beidou's Burst's coordinated attacks and Electro application provide huge DPS increase to the team. Beidou however needs another Electro teammate to keep her Burst active for as much time as possible.
Thoma's role is to prevent Yanfei's interruptions with his shield. Thoma's value here is his being Pyro, which means Pyro resonance and extra ATK for the team, which is very valuable when transformative reactions are used. However Thoma might me a niche character to build and use, so any other shielder does the job too.
Team #3: Tankfei
NOTE: NEEDS CONSTELLATION 4 OR HIGHER
Support Yanfei comes with Pyro resonance, either occasional healing (Prototype Amber) or ATK buff (Thrilling Tales of Dragon Slayers), energy generation, a shield and Pyro application to the team. Her role here is only to provide utility in exchange of a DPS loss.
Kazuha benefits from Yanfei's Pyro application, enabling Pyro swirl. Hu Tao has limited Pyro application so in order to profit from the 4pc Viridescent Venerer and other Anemo supports' effects she requires a second Pyro unit on the team, who can apply Pyro on her behalf.