Sanji has the best writing in the series, and that's even with FMI being trash and a lack of fights.
Punk Hazard - he met Kin'emon and saved Marines
Dressrosa - he converted Viola and saved his crew from Doffy's trap
Zou - led the "Twirlyhat Pirates" and rescued Zou
WCI - massive second arc that set up even more mysteries
Wano - more development, decent clashes, enormous powerups
Egghead - MVP with insane feats in speed, versus Seraphims, Kizaru and Elders
Elbaph - EVEN MORE DEVELOPMENT, Gaban reintroduction to the story, clashes with Imu and awakening Conqueror's
Oda has always favoured Sanji with good, impactful writing.