Update: now there's this https://channel3.gg/changing-the-game-superpost

This has sent me into an existential crisis, there are too many good and correct answers. I have not seen this answer yet though: Game Gear. I've said before my parents got it for me and I didn't know it even existed. I had played a Game Boy at the dentist's office. Great to see, but the green/black screen was what it was. My mind was blown turning this on and seeing Sonic in my hand in full color.
Honorable mentions:
Any system the first time I turned it on.
Switch and Breath of the Wild - The power of the sun in the palm of my hand -- I had given up on Nintendo but they pulled me back in
Mario 64 - I had worn out the demo VHS tape sent to me, but it didn't do justice when I turned it on and ran to the castle for the first time, entered the Bob-omb battlefield, any of it. Mindblowing.
PS3 - The first time turning it on with a truly functional controller that was wireless without compromise. The quality of the video jumping over the PS2 was mind-blowing. The 4 and 5 have been diminishing returns. The 3 was so clean, so elegant loading up.
I'll stop there. I could do this forever. I may have to do a personal super post of this someday. I need to just write super posts I have in my head for the rest of my life.