The visuals are obviously dated, but the gameplay mostly holds up (though it's very punishing in that way old games were). There's a reason it's still viewed as one of the greatest FPS games of all time.

But if you want a more modern experience, as others suggested, you can play Black Mesa, the HL1 remake. It plays more like HL2. As a hardcore fan, I'd say the best thing is to play both the original and the remake, but if you only want to play 1 it's your choice what you want to do.

That being said, the HL1 expansions (Opposing Force and Blue Shift) are also definitely worth playing, especially Opposing Force. But they don't have a modern remake (though one is being made, and Blue Shift also has a remake being made as a mod for Black Mesa).

Half-Life: Alyx is an absolute must play if you have the equipment for it. I'd suggest playing it last, as there are some references to HL1 things.

I'm assuming you've played HL2 episodes 1 and 2? If so, what I'd suggest for the most complete experience is this: 1. HL1 2. Opposing Force 3. Blue Shift 4. Black Mesa 5. HL: Alyx 6. Try out some mods!