Half Life 2 still holds up really well, honestly better than a ton of modern first person shooters. The only places it's lacking from a non-technical aspect is enemy variety. If valve did a remake just updating the graphics and gun play that would be my only knock on it and that says a lot for a 20 year old game.
The physics puzzles, whole still functioning fine, feel extremely goofy today. It's easy to forget how revolutionary they were. I'm not saying the game shouldn't have them or anything, but some are just so silly lol.
On enemy variety, I see the critique of games like Zelda: BOTW and even realistic games like Hitman. Something those games have in common is very well-made enemy AI that presents you many ways to defeat them.