[2023] What is the best stat to upgrade?

Tbh it's incredibly down to user preference. But general user sentiment is that Strength and Intelligence are OP in their own ways- For general playing, Strength is a better option between the two, as Int's combat style is weak early into the game.

General sentiment aside, Pick the main stat that has the weapon types you like most- Like shotguns? Go Strength. Prefer assault rifles? Go Fortitude. You're also honestly going to want to spread points around to pick up minor abilities sooner or later, so you'll end up with points in each tree, with your combat tree taking the lead.

That said, I personally have played at least one playthrough focused purely on each tree, and while their playstyles and approach are different, I never really felt any of them to be wanting. The way you have to play each tree IS different though.

If you want my advice, don’t outsource the fun of picking a character build to all of us! Enjoy exploring for yourself what perks & skills allow you to do. If you don’t like how it plays out, you can use Grandpa’s Forgettin’ Elixir to reset, or start a new game.