Probably any of the ones that don't provide some sort of benefit to their evolved forms...which is most of them. However I like most of the baby Pokemon, and some of them I like more than their evolved forms. So if I had to choose one....uh, Happiny? I dunno.
It'd be cool if the babies were stuck on earlier routes rather than restricted to later routes. B/W2 were good about this, in regards to Azurill, Magby and Elekid, as were the gen 4 games and their use of Budew, but then those games also stuck Mantyke by the last gym. :/
But yeah, I guess in terms of design and potential usefulness...it would probably Happiny, even if I actually do like it. That, or Smoochum. I probably like Smoochum a little less but if I'm gonna give Magby and Elekid a pass, I guess it deserves one too?
I chose smoochum but azurill sucks as well, I'm trying to use one in white 2, level 18 or something and still hasn't evolved yet. Can't wait for some stab so I can at least do SOME damage to enemies. Although I haven't checked its nature yet, that could be an issue too...
Most of them are awful. Like Bonsly. Why did Sudowoodo need a prevo? What benefit did he get from that? "An evo? Don't be stupid! Let's give him a prevo!"
Absolutely. Most of the pokemon that got them are rolling around in NU where they aren't even viable there, and their chances of getting another evo were blown by getting an even more worthless version of theirselves. Heck, the baby forms even prevent stuff like clefairy and pikachu from participating in LC.