Hmm...I like them all really, for different reasons. Although LeafGreen just seems boring...I dunno if the originals had an intro or not, but I think as it's basically going to the beginning it doesn't need one - it should go straight from the "Gamefreak 19?? - 20??" part to the main screen with Venusaur.
Platinum's always looks nice as an improvement over Diamond. Emerald's was the first one I saw, and is probably still up there. But overall I think HeartGold's is the best. Nice colours, shows you all you already knew about the game whilst building up your expectations, and decent music too.