(VERY LONG LIST AHEAD!)
I'll give my favourites from each gen:
Gen 1: Bulbasaur line, Gengar, Hypno, Cubone, Lapras, Jolteon, Articuno
Gen 2: Totodile line, Ledyba, Spinarak, Crobat, Ampharos, Politoed, Umbreon, Murkrow, Shuckle, Heracross, Ursaring, Skarmory, Smeargle, Hitmontop, Elekid...