Gosh, I dunno, either season 1, 2, or 3.. The first was just, the first.. I dunno.. Plus its also the longest season.. Right? But the second one they didn't need Brock's cooking skills, just island fruits, those were pretty good times though, with islands and all.. And the gyms were different from most gyms..
And yet, the third one was always most fun for me for some reason XD... Probably because it was when they started showing a new generation of Pokemon, more then Togepi and Ho-oh I mean.