Tetris for the good old Gameboy. That may just be one of my earliest memories, I think (I was very young) my mom had to do jury duty or something and I had to come along, and I was so bored I cried (don't remember but I think I did) and my mom pulled out a GBC with this game. I didn't understand it, but I loved it.
Although right now my favorite version is Tetriabetes, a homebrew on the PSP by Dragula96. Awesome music, random bag (no more waiting 2 decades for a long piece), hold pieces, cool sound effects, one of the best PSP homebrew imo.