Jeez... hard question, he was soooo prolific--40+ novels, 26 of which were adapted to movie and tv. He didn't write series, but some of his characters reappear in subsequent novels. Leonard wrote in two genres (westerns, originally; and later crime). My advice is go here and peruse:
https://en.wikipedia.org/wiki/Elmore_Leonard.
I read many of his novels (not the westerns; don't know why); of those that I remember, I'd recommend GET SHORTY, BE COOL, RUM PUNCH, OUT OF SIGHT. And I'd vote
against CUBA LIBRE and TISHOMINGO BLUES, but ymmv.
The guy was a master story teller.
Enjoy.