Finneon and Lumineon without a doubt. They're probably the most forgettable mons ever made. Let's be honest here, who actually remembers or uses one of those generic fish mons?
But if we want to look at this in a different light and choose a mon that's actually relevant, I would have to say Probopass. I mean, it's kinda...I don't know how to describe it. It feels pretty forgettable, too. All it's pretty much good for is bulk, even with Sturdy's buff, but for the sake of argument, let's only look at this from a Gen 4 perspective, back when Sturdy was next to useless. Along with that, did Nosepass really need an evolution a generation after it's introduction? Sure, Tangela, Togetic, and Sneasel got them too, but they're all actually really good and improve those three mons (even though their weaknesses don't change until Togekiss in Gen 6).
Now, enough bashing Probopass. As other people above have mentioned, Phione and Male Combee shouldn't exist either. Combee is probably the best example. Let me go over a backstory of back in the day of Diamond and Pearl. I caught a Combee and raised it up to use it in Battle Revolution because I thought it would eventually evolve into Vespiquen. Trust me, you don't want to know how salty I was when I realized that male Combees don't evolve at all.
