I haven't played Half-Life: Alyx yet, I'm sure if I had I would pick that (though I only played the HL2 demo in 2004, didn't finish the actual game until 2011).
So I would maybe pick Robo Recall from games I've actually played. That moment in the opening when all the robots turn to look at you blew my mind on my Rift CV1 when I first played it in 2017. I haven't played it that much since, with Beat Saber definitely being the game I've spent most VR time in, but in terms of what I imagined the future of gaming to be, that is pretty much it. I love Rez Infinite but I didn't play the original Rez until 2007, so it wouldn't have as much impact as showing 2007 me.
For non-VR, maybe Yakuza Kiwami 2? I prefer 0 as a game but being able to go indoors/outdoors without loading would have blown my mind, along with the fidelity of the character models and the wealth of minigames. I was also really into stealth games at the time because of Splinter Cell, so as a runner-up maybe Deus Ex: Mankind Divided (I always play these games going the stealth route)?
I wouldn't show him Shenmue III, I'd let him keep dreaming.