Gen 5 by far. While I think many of the gen 2 designs were bland, I for the most part enjoyed all of the designs for every gen except gen 5. People keep saying "oh that's just the Pokemon cycle", but I disliked the gen 5 designs when they were first announced, I still disliked them as I played through the game, just waiting for them to "grow on me" like everyone kept saying, and I still dislike them. On the other hand, I never had problems with any of the other gen designs. I dunno, I just feel like a lot of them look like derpy caveman drawings. I can count with my fingers how many of the designs I actually liked.
IMO generation 1 had some of the ugliest designs, so don't get me wrong I love the first generation, I grew up loving it and what not...but I'm not blind, ppl tend to make it seem like the greatest because they can't get over their nostalgia , but ..im not blind how is a pile of sludge or an upside down pokeball creative or cool, and lets not bring up jynkx or Mr.mime most hideous designs, not throwing shade but I'd rather take an ice cream cone anyday over this two