>>46929380A lot of gen 3 tops the list (top 50) (not in any order): treecko, blaziken, breloom, medicham, altaria, flygon, dusclops, milotic, tropius, metagross, rayquaza; then gen 2 mons like all the gen 2 legendaries/mythical, xatu, heracross, kingdra, mantine, espeon/umbreon, tyranitar, crobat; and then gen 1s like kangaskhan, dragonair, gengar, lapras, ninetales/arcanine, zapdos, vaporeon, pidgeot, tentacruel, scyther, venomoth, victreebell. After that it's pretty lightly scattered and tapers off across the newer gens
tl;dr
Number one is really hard to pick but I guess I'd give it to pick related. Metagross, lugia, treecko, and rayquaza all pretty much tie with it for first though