Watch it until the 3rd gen is over cos that's when it started getting really dumb. I'd watch Kanto there's some really funny episodes in there, Orange Islands is ok, and Johto is ok. Ash gets a Larvitar and it always cries. It's so epic xD
Not to mention the first episode. Hands down one of the funniest ever moments ever! Ash is riding bike he stole from Misty taking Pikachu to Poke Center and he says, "Hang on Pikachu, were almost there" then he wipes out. Haahaha It was so hilarious.