It's tied between Gen 1 and 5 for me. Gen 1 was my first gen and I have great memories of it. However, I took a long break from Pokemon, and when I returned I played Black 2 and absolutely loved it. I honestly can't choose, I love them both. I do believe Gen 6 will become my favorite though.
https://www.youtube.com/watch?v=wNsSfiTm5yI My Nintendo 3DS Friend Code: 0173-1411-9831