Leonard's novels are stand-alone, even in the few which share characters, so it really doesn't matter where you start.
You'll find his writing much leaner and down-to-earth than Crichton's or Flynn's. Leonard doesn't spend much time on descriptions; his forte is characterization through dialog. If you like Hemingway you'll like Leonard.
I first started reading Leonard after watching the film 3:10 to Yuma, which was based upon one of Leonard's short story westerns. A LOT of his stories have been made into movies and TV shows.
I'd have to say that my favorite is novel of his is Out of Sight.
I remember reading some Leonard book several years ago... Bought it for a quarter at a garage sale. I liked it, but... never read anymore.
I also REALLY LOVED hearing Vince on Hugh Hewitt's show, and others... He seemed like a straight up great person, besides being a fantastic author.