You guys are overcomplicating this. It's not a continuity-heavy series. It's a vibe. Find the vibe. For me, the vibe is a diner, a coffee, a quiet stranger, and a whole lot of impending violence. Which book has that? All of them. So close your eyes and pick.
Confusion about the order is super common because of the way it's published. You've got the original series timeline, which is different from the publication order for the first few books. 'Killing Floor' is the first novel Lee Child wrote, so that's your true starting point. But chronologically, 'The Enemy' is a prequel set when Reacher's still in the Army. My advice? Go with publication order. You meet him as a drifter right away, and the flashbacks in later books fill in the backstory more effectively. Starting with the prequel first kinda ruins the mystery of his past. The publication order is the way the author intended the character to be revealed to readers.
After the first dozen or so, they're mostly standalone adventures anyway, so order matters less.
Just a heads-up, the writing style shifts noticeably after the first few books. The early ones have more of a hard-boiled, almost poetic minimalism. Around book ten or so, they become more streamlined and cinematic. Some fans prefer the early style, others the later. Reading in order lets you decide which 'era' you like best.