My first game was Red. I still enjoy playing it. I actually played through it a few months ago (when the VC releases came out), and I played through Blue then as well. I might start them again and play them again sometime soon.
There's something that I like about the simplicity of the gen I mechanics. Sure, I love the newer games too, but a lot of my favorite Pokemon are in gen I, and I feel like I now have a much better grasp on what set gen I battles apart from future generations than I did when I was younger. I also find that the single special stat sometimes allows me to use Pokemon that I wouldn't normally use today, or use them in a different way, so that's a lot of fun, too. And Red brings back so many memories.
(Now, if only they would release GSC, my favorites, again! And Stadium for my now competitively-made gen I teams!)