4th >= 2nd >= 3rd >>>> 1st
4th, 2nd and 3rd are awesome generations, with very creative Pokémon and interesting added things, originality is an word that categorizes all those three generations IMO.
2nd had a lot of interesting Pokémon, Noctowl, Furret, Totodile, Cyndaquil, Chicorita, Scizor, Heracross, Ursaring, Murkrow, Kingdra, Porygon2 (^^), Miltank, Xatu, Ampharos, Lanturn, Ariados, Sneasel, Houndoom, Raikou, Suicune, Ho-oh and Lugia (Lol, pratically all of them xD). 2nd Gen added time based evolutions, happiness evolutions, trade+item evolutions and Stat based evolutions. In the 2nd gen we got the Special split and a new stone, pratically useless, but a nice one!
3rd showed us the abilities, natures, EV training, Base stats, Breeding improvment, and later in Emerald a 2nd effect for abilities. 3rd gen added a lot of creative Pokémon, such as: Masquerain, Breloom, Ninjask and his brother Shedinja, Torkoal, Grumpig, Flygon, Lunatone, Solrock, Claydol, Cradly, Armaldo, Castform, Bannete, Dusclops, Tropius, Glalie, Spheal, Walrein, Huntail and Gorebyss, Metagross, The Regis and the multiform Deoxys, another thing added in the 3rd gen.
As for 4th creative starters (monkey has some negative points, but it looks cool in a video I saw) if not the most creative starters, because we didn´t saw an Ice Penguin, an Water Turtle or a Fighting Monkey... oh wait, we saw that... it could be Fire/Psychic... 4th gen added attack based evolution, Hold item happiness evolution, gender evolutions, party based evolution, and a lot of new things you saw in the main site. It also added a breedable Legendary, eggs hatching in the obvious Lv 1. Creative Pokpemon now? Well, I´d write them if their names were easier to memorize and write >_< but the ones I can say are: Electric/Ghost ; Flying/Ghost ; the fossils, Kirlia alt. evo, the starters, and lots others!
wrote a lot!