ugh gen 4 was the worst! Except for Drapion, Lucario, Garchomp, Staraptor the rest of the non starters were terrible. Worst gen for me, which is a shame since I actually liked Shinnoh second best as an actual region. But so many mediocre to bad new pokemon.
I know this isn't supposed to be a "which Gen you hate the most" thread, but I personally dislike the designs in Gen V the most (and I actually love the Ice Cream pokemon!). I didn't like any of the evolutionary lines for Tepig, Patrat, Lillipup, Pansage, Pansear, Panpour, Woobat, Drilbur, Timburr, Tympole, Venipede, Darumaka, Scraggy, Gothita, Foongus, and Deino. For singular Pokemon there's Throh, Sawk, Druddigon, Heatmor, and the three musketeers, which I just completely dislike.
Needless to say, Gen V and I don't really mix well, but maybe that was just my "falling away" generation.
Ultimately though, Gen VI turned things around for me immensely, and I love the majority of the designs this gen too. I know people will say "I wish we had more pokemon like in Gen V!", but I'd personally prefer smaller Gens like these where their designs are able to be more focused and (in my opinion) more well thought out.