
I started with 0 and given the trajectory of the series, I still say that's ideal for starting the story and familiarizing yourself with the characters. Playing Yakuza Kiwami 1 may feel like a step back gameplay-wise at that point, but Kiwami 2 more than makes up for it.
I could see an argument for starting with Kiwami 1 and playing the series technically in order, with 0 as the actual 6th entry before Yakuza 6. The gameplay and available amount of sidequests might feel more consistent that way.
Basically, in my mind, where to start depends on whether you'd like to play the series in order of release (so 1-5, 0, then 6) or in order of story (0-6).
And of course, no harm no foul if it turns out you're not a fan of the Yakuza formula. Won't change my personal take that it's the greatest form of art ever made, but that doesn't mean it's for everyone. I started out by renting Yakuza 0 via GameFly.