Just one? Well I guess that rules out the ultra beasts sadly if I can only eliminate one. Gumshoos then. Stop trading these things to me, nothing is more traumatic. Why ruin a mongoose with this stupid toupee? And what are you going to do with this thing in battle?
Only one Pokemon? In that case, I pick Emolga, because I find it very annoying, and I'm tired of Game Freak shoving it in every regional dex. Also, since it's a stand-alone Pokemon, I don't have to feel bad for breaking the one Pokemon only rule.
There are quite a few species of Pokemon that I've always found unnecessary, but right now I'm feeling cynical about the Riolu line since I've always found Lucario in particular to be highly overexposed and generally overrated.