I believe generation 5 has some of the coolest designs in the series. My favorites include Volcarona, Hydreigon, Whimsicott, and Lilligant. Some are perhaps even over-designed, such as Scolipede, Escavalier, Galvantula, Sigilyph, Reuniclus, Chandelure, Eelektross, Haxorus, Heatmor, Braviary, Golurk, etc. These Pokémon have so many colors, spikes, spots, lines, and other markings. You wouldn't dream of seeing these kind of markings on generation 1 Pokémon. The originals were more simple with choice of color pallet, for example, Pinsir, Scyther, Alakazam, Gengar, Electrode, Dugtrio, Onix. I imagine the limitation was a result of RBY sprite color issues; however, some generation 1 Pokémon were re-colored in later generations, such as Ivysaur.
To each their own! There are plenty of generation 1 and 5 Pokémon that I adore. For example, Dewgong... So boring, yet so (icy) cool! Everyone is allowed an opinion on which Pokémon (and generation) are their favorites. There are 1,000 Pokémon over 9 generations to choose from.