I liked 5 best because I was getting a little disillusioned with the series. I started play Blue when I was 8 and it came out, loved it, played she shit out of it, played it legit and glitched the shit out of it, finished it multiple times, basically fell in love with the game. Then came gen 2, me and my brother got Silver and Gold, respectively, which I loved even more. Played it so many times, got a legit shiny (not just red Gyarados) and didn't even realise what it was, battled with so many friends and my brother and his friends, Gen 3 I didn't play right away, wasn't playing a lot of games, but I did play it before Gen 4 came out. Didn't love the map, or the characters, or the new Pokemon so much. Played Ruby once and Emerald once. Never got the same feel as Gens 1 and 2. 4 brought me back into the series with some cool new Pokemon, especially hitting me in the nostalgia with some new evolutions to old Pokemon and it's when I really got into the competitive aspects of the game. I also liked the map and the story and the training was facilitated. It cemented it for me when HG and SS came out, I really loved Gen 4 then.
But Gen 5, Gen 5 is the greatest. It rebooted the series, forced me to learn the new Pokemon and use them, it was a new region, it was pretty, I liked nearly all the new Pokemon, the story was great, the training is great, they made great additions and changes to training and breeding and TMs and such. The Battle Subway was great. I really just loved Gen 5.
And in order I would say:
Gen 5
Gen 2
Gen 1
Gen 4
Gen 3
And I liked them all lots except Gen 3
