OnGuard Akita 8043 7' x 10mm cable, U-Haul disc lock (leftover from a rental storage unit) with very short shackle. I coil it up and lock it under the saddle, with a Velcro strap secured to the seat post to keep it from flopping around. I doubt this rig would take more than a minute to cut through with a decent bolt cutter but I used a much lighter weight cable and lock for years and never had a theft attempt.
The real trick is to find a lockup location that discourages theft. I've seen videos of thieves in the middle of NYC and London brazenly wielding angle grinders and huge bolt cutters while pedestrians passed without a second glance. But that wouldn't go unnoticed everywhere. Probably also helps to disguise it as something nobody wants. Like a flip phone. Or a sequel to The Last Airbender. Or fruit cake.