I do wanna chime in on the DPPt Lucario topic from several years back. I used Riolu on my first run, and it actually wasn't too bad. Aura Sphere comes at level 37, which isn't actually too massive a jump past Byron's levels when you can reach Iron Island. Once Riolu evolves, you can give him Focus Blast from Veilstone, Brick Break from Oreburgh gate if that isn't overly contested for your team, Swords Dance by level up for sweeping bosses (no TM detour), Drain Punch from Maylene, Flash Cannon from Byron himself, and a couple elemental punches plus Zen Headbutt (for Poisons like Crobat) or such from Pt move tutors (though I was playing Pearl).
The main obstacle is babying it to evolve, but once it gets to that second stage, it has a fairly good TM movepool for handling the next couple of fights at least as a glass cannon (so maybe some Speed or a little level leg up helps for some faster mons like Weavile), and if you're willing to bring it up to the leveling curve anyway it gets Aura Sphere and Close Combat in relatively quick succession, giving it options as a mixed attacker or simply picking a side. I would say it's at least worth "Magikarp" credit, with a long babying period that has a fairly good payoff, albeit both instances are probably less dramatic since Riolu doesn't have a specific level to evolve and Lucario is good without being quite the level of something like Gen 1 Gyarados once it evolves.
Regular Lucario itself doesn't have much place in OU (Mega is just as OP in game as competitive) that I can see, as it falls short of "sweeper" tier speed and it has some coverage issues for Wall Breaking (albeit Meteor Mash now helps its dual STAB accuracy)
I'm not sure what qualifies as bad for competitive (having a place in no tier or being relegated to a particular tier), but another mon that springs to mind for me is Darumaka, especially in Gen 5's games, especially if N's Pokemon are valid for boosted EXP. In competitive, being a a glass cannon wall breaker relying heavily on a Recoil STAB does hurt its performance, especially with hazard weaknesses. In-game, however, the speed is fine, and it comes with Fire Fang and very close to upgrading to Fire Punch (if not having it outright for N's at level 22), plus some Fighting coverage and upgrading to Flare Blitz for a Nuke option. The Coverage of Fire/fighting and maybe a Rock type TM is enough to double down for a nuke and "sweeping" move, and as noted the boosted EXP definitely helps its leveling curve.