I've grown a lot less critical of Gen 5's Pokemon over the years, with Solosis and Vanilite's line being notable examples. That being said, there are still a few that I can list here
Basculine: This fish isn't horrible, but as many people have already stated, it is bland and forgettable. I also feel they could have given both colours a similar, yet separate, evo. This would have highlighted their rivalry (they could have evolved to gain the upper hand on the other), made them more memorable, and allowed them to be viable team additions.
Marcactus: I had forgotten about it until I read through this thread, which speaks to how little impact it had on me. For the record, I've never forgotten the likes of Quilfish and Girafarig...
Pignite: When I first saw Tepig, I thought it would be my starter, hands down. It's cute, unique, and seemed to ooze potential. Seeing its evos, however, ruined the fire pig for me. I'm no fan of Emboar's, don't get me wrong, but it at least looks like a threat. Pignite on the other hand is just lackluster and unappealing to me. I think the fact that it looks like its wearing overalls may have something to do with it, but its design overall is weak in my books. Being disappointed by both Pignite and Emboar, Tepig became my last choice... (interestingly enough, the opposite happened with Oshawott)
I also dislike Stunfisk's cry, but I've become more neutral to the actual Pokemon. I must also mention Garbador, and although its design is still cringe-worthy, I don't hate it as much anymore. Having watched a play-through of Reborn is responsible for my slight change in opinion.
I think that's all cx