Here's my most hated 4th Gen Pokemon.
Purugly: I did like Glameow, but Purugly really lives up to its name, cos it is ugly. At least Mars's Purugly can be really tough given how early you first fight it.
Wormadam: This thing is absolute rubbish, shallow movepool, terrible stats and awful design. I hated the entire evolution family but Wormadam especially.
Bibarel: It may have unique typing, but its stats are horrid. Makes a great HM slave though.
Kricketot: This thing takes ages for me to train despite evolving early, and its only stuck with 3 level-up moves. I hated Kricketune as well cos of its design.
Heatran: It may have a niche in competitive battling but I hated its design.
Tbh there weren't as many 4th Gen Pokemon I hated as I thought.