Magikarp, I can't call useless. They had to create a weak pokemon, since things would be way too easy if you started off with gyara in his full form, especially since he gets so good (taunt-rados... *shrudders*)
No. I would have to say all the baby versions of pokemon that they have added in 4th gen as they are a pain to get, a pain to evolve, and do not contribute anything to the move-pools of the final evolutions.
What I mean is that the older baby pokemon had specific contributions to evolutions.
Take, for instance, Pichu. While that can be skipped completely while breeding and training for a Raichu, it contributes several very important moves that will be very helpful later on such as Nasty Plot through leveling or Volt Tackle through breeding.
And now, as opposed to that, look at Bonsly. If you look at the movepool for that versus Sudowoodo, the only move that's any different is Fake Tears, which... Looking at Sudowoodo's base stats, is completely useless, as no one in their right mind would put special moves on a pokemon with 30 base Sp.Atk while that same pokemon has 100 base Attk.
So, my vote is with the baby "evolutions" that were added in 4th gen. I rest my case.