Starfox
Hero
Mathfinder
It's definitely math intensive, but there's just so much to love about Pathfinder that the math is worth it.
Currently I'm unwilling to play 5E without bringing in lots of stuff to Pathfinder. And, earlier this week I started thinking, wouldn't it be easier to import what I like from 5E to PF, rather than the other way around? Afterall, there's a lot of math involved in almost every conversion from either system to the other, but there's a lot more Pathfinder than there is 5E. So, currently leaning toward PF as my base.
Starfinder seems to be a streamlined Pathfinder. From day zero, Starfinder was accused of being Pathfinder 2.0 (or d20 3.75 if you prefer). Paizo repeatedly claim its not. But some Starfinder rules can definitely be used in Pathfinder, here are some examples.
* Flat-Footed is a -2 penalty to AC.
* Attacks of opportunity happen *after* what triggers them. No more concentration checks for attacks, no more withdraw action.