I like the original black & white Gameboy version. To me, that is tetris, portable, and really the first one that I played. Whenever the itch strikes, that is the one I run back to. Although the first time I player it the colors were pea green, I prefer to play it on my Gameboy Pocket, which I had out yesterday playing Nemesis.
The original Tetris on GameBoy has always been my favorite and continues to be one of the all time GB classics. I would count Tetris 2+BomBliss on Famicom where BomBliss plays exactly like Tetris Blast, so it is my 2nd favorite because it beats the crap out of NOA's NES Tetris which only had 1 player modes and Tetris 2+BomBliss saves!