I was a completionist from birth. I beat Super Mario Bros. when I was 3 (yes, even "hard" star mode where goombas are the black-shelled guys), and would play it endlessly. Then again, I'm probably a bit older than most youtubers out there, so I grew up right around when the Nes was still new.
First game I LOVED was probably Phantasy Star 4: End of The Millenium (a 1993 RPG)...love it so much it became my first series on my channel. It's what really made me fall in love with RPGs, and makes me more critical about the story in said genre (sorry, but FF7 is terrible in terms of story...even 8 does a better job of storytelling...and I don't wanna talk about FF10).