Stage 1: Oshawott - I don't think there's any starter basic form I actually dislike, but Oshawott is the ugliest of the lot in my opinion. To me, it kind of looks like a weird cross between Bidoof, a snowman and Piplup. I'm glad Dewott looks so much better.
Stage 2: Quilladin - This one was difficult to choose. Croconaw is ugly and ignores most of the likeable features of its predecessor's design, Combusken looks very odd next to Torchic and Blaziken, and Servine is just a bigger Snivy. All 3 of the Sinnoh starters mid-stages look like they're going through their awkward middle school phase too. Despite all of this, the award has to ultimately go to Quilladin for being absolutely hideous.
Stage 3: Sceptile - Again, there's a lot of final forms I'm not a fan of. Venusaur is just an uglier, fatter Ivysaur. Samurott is a huge design leap which makes no sense to me. I remember picking Piplup as a kid and being absolutely devastated when it evolved into Empoleon, as it was by far the least cool of the final evolutions in that generation to me, but it's grown on me since. Chesnaught seems to be super divisive, and I'm on the side of not liking it, but I don't think it's completely awful either. I'm not a huge fan of Dartrix becoming a ghost type upon evolution, but I don't dislike Decidueye's design. But with that all out of the way, the worst offenders of bad final evolution design are the gen 3 starters. Blaziken suffers from the same problem as Samurott, in my opinion. Swampert is just a big ugly Marshtomp, which is sad because I LOVE Mudkip and Marshtomp. And the worst of the lot is Sceptile. I don't know, he looks ugly and super smug. Just overall has a very punchable face.