Onix. Yeah, Onix. He doesn't just need it, he deserves it.
There are two Pokemon who evolve via trade w/ Metal Coat: Scyther and Onix. Their evolved forms, Scizor and Steelix, are Steel types designed to be stronger but slower, and have the same BST, as their pre-evos. With that in mind:
Scizor: 500 BST Scyther: 500 BST
Steelix: 510 BST (The difference is so small as to not matter.) Onix: ...385 BST
Onix is far, far weaker than Steelix and even Scyther. It would need 125 more BST to be on the same level... so the solution is simple: give him 25 to everything but Defense. (It doesn't need to be any higher.) So, now Onix has,
60 HP 70 ATK 160 DEF 55 SP ATK 70 SP DEF 95 SPD
It's a simple yet perfect change. Now, while it won't go take the game by storm, it's usable. And it is to Steelix what Scyther is to Scizor: worse Attack and Defense (and typing) but better Speed. With high Defense, moderate Speed, and Evolite, but mediocre HP and bad defensive typing, Onix will be a good but not broken wall and support Pokemon. It won't be better than its evo, but it will be usable in its own right, like Scyther.
Please, GameFreak? I might be the only one in the world, but I love that rock snake. Buff Onix, for me, and for his other two or so fans, including Brock.