Gen 5 is easily my favorite generation, and i think it did my favorite type (Ghost) justice. Golurk, Litwick, and Chandrelure are my top 3 favorite Pokemon period, and Yamask, Cofagrigus, Lampent, and Golett are also good designs.
For say, Steel types, I think gen 3 did them the best. Registeel, Aggron, Aron, and Mawile are all great, and Lairon is ok. All of them are 8/10s at worst.
What region do you think has the best designs of each type? Like for example my incomplete list would be
Ghost: Gen 5
Steel: Gen 3
Flying:?
Grass:?
Fire:?
you get the idea.