KE+KM combo is close to the best, KE allows you working around yet another dumb limitation of the OS of treating left and right the same (otherwise Ukelele is nice), KM gives you a more friendly GUI and integration with other apps (like Alt+F)
(the only missing low-level part in KE is stuff like home row mods where you'll need another utility like kanata/kmonad, but not sure how well they integrate with KE)
I'm currently using Ukelele, but it's not ideal. And I would really really like to remap Right Alt different to Left Alt, to get a proper ISO layout!