You're hitting on a genre that's exploded lately, and honestly, the definition of 'best' depends so much on what kind of action you crave. Is it the physical, bone-crunching kind or more of a strategic, political battlefield? My absolute cornerstone recommendation has to be the 'Mistborn' trilogy by Brandon Sanderson. Vin's arc from a scared street urchin to a figure reshaping an empire is phenomenal. The Allomancy magic system is essentially a built-in excuse for incredible, creative fight sequences, and the way her trust issues and resilience are woven into the plot makes the action feel earned, not just decorative.
For something with a completely different texture, N.K. Jemisin's 'The Fifth Season' features Essun, a woman whose personal tragedy unlocks world-ending power. The action here is less about choreographed duels and more about the raw, geologic force of emotion and survival. It's brutal and breathtaking. On the more fun, globe-trotting side, I re-read the 'In Death' series by J.D. Robb when I want a procedural thrill. Eve Dallas is a NYPSD lieutenant in the mid-21st century, and her action is gritty, detective work punctuated by intense confrontations. She's deeply flawed, brilliantly sharp, and the long-running series lets you see every facet of her toughness develop over fifty-plus books. A newer voice I'm excited about is Fonda Lee's 'Jade City' series. While it follows a family, the women like Shae and Wen are absolute forces, maneuvering through corporate-style clan warfare where the magical jade-fueled combat is both brutally intimate and strategically vast.
Don't sleep on some classic urban fantasy either. Ilona Andrews' 'Kate Daniels' series starts with a scrappy mercenary in a magic-destroyed Atlanta and builds to wars between gods. The blend of sarcasm, mythology, and sheer physical tenacity is a blueprint for the genre. I find myself circling back to these characters not just for the fights, but because their strength feels multifaceted, born from vulnerability as much as skill.