Probably Gen 2. The only Gen 1 Legendary I like tremendously would be Mewtwo, as well as maybe Zapdos due to its great competitive use and typing. Meanwhile, Gen 2 had such competitively viable Pokemon as Lugia and Ho-Oh, something with as kewl a design as Raikou and generally provided a lot more interest when it came to in-series lore than with the Legendary Birds (Mew does have a lot of that, though it isn't as important for the main story).