The best one is the one that was pampered.
I have a series 11, 16, & 20; I like the 11 best. It has a Kohler engine that has been very dependable. Newer models have more safety features, but I prefer the simpler controls on the older ones. I don’t think you can go wrong with any of them. The mechanicals aren’t hard to master, and I’ve never had any problems getting any part for those three machines.
I bought a series 16 with a Kohler engine that was run without oil and locked up. I got it broke loose and nursed it back in service. It ran for another year before it threw a rod; I suspect a tenant didn’t check the oil.