Well 3rd gen is the cutoff since 1st and 2nd gen aren't compatible with future gens. 3rd gen has certain move tutor moves like Counter, Seismic Toss and Body Slam. For Snorlax/Mewtwo/Wailord that's how you get Self Destruct. While Snorlax can get it as an egg move from a Pokewalker Munchlax that would be the only egg move it could have while a 3rd gen could get Curse egg move, and have it taught Counter/Self Destruct/Double-Edge. There are some other egg move combos that require 3rd gen like Starly w/ Double-Edge/Roost/Pursuit, Gligar w/ Counter/Baton Pass but you don't need to RNG in 3rd gen for them, only need the right parents from 3rd gen.