Earlier quoted context omitted.
It seems quite a lot of people around here have a thing for El Capitan. Coincidentally this is the last one called "OS X".
Confirm. El Capitan is the last good and stable version.
As for 10.13. If an OS changes filesystem, its often recommended to wait (same with in the past e.g. FAT32 -> NTFS, Ext2FS -> Ext3FS, Ext3FS -> Ext4FS although IMO the latter two went flawless for me).
The reason I went for it -even though benchmarks of encryption + APFS specifically were abysmal compared to encryption + HFS+ whereas no encryption + APFS or no encryption + HFS+ were acceptable differences)- is because it was already tested on iOS.