For me, it's the Gameboy Color.
My Gameboy obsession started with the original DMG-001. I would skip the Play It Loud re-case-colorings and the Gameboy Pocket models and went straight to a Gameboy Color (I wanted the Atomic Purple, but it was sold out everywhere at the time, so I got one in Dandelion).
While I have since played Gameboy carts on a Gameboy Pocket, a GBA, and a GBA SP, the Gameboy Color seems to hit the sweet spot for the handheld feel (the GBC just feels good in your hands). The GBC-exclusive games are, of course, the reason for the system, and my copy of Super Mario Bros Deluxe was always with me. The optional re-colorizing of black & white games is a nice bonus, and the hybrid black-carts with bonus features when played on a GBC make it even more fun.
Side Note: I own a Super Gameboy cart for my SNES and a Gameboy Player for my GCN. Both make playing Gameboy games fun on the big screen, but there's there's something about playing these on a handheld that make them more enjoyable for me.
Anyway, Voat, which Nintendo-made Gameboy system (compatible with original Gameboy carts) is your favorite? and why?