"Over-hyped" =/= bad. Don't be a Poke-hipster.
Lucario is one of my favorite Pokemon because how versatile and hefty they are. Steel is a great type in-general and Fighting types are the hard-hitters of the franchise. It always feels like an honor to use Lucario since they're one of the few rare Pokemon that do fantastically in battle.
Greninja is pretty awesome too. Like, ninja frog, okay? Protean and Water typing and easy access through being a starter... Nothing wrong with Greninja. Honestly, I think the design's a little off-putting. Froakie and Frogadier's designs were based around bubbles and foam, yet Greninja has none and their scarf is now their tongue? Though, to be fair, Lucario also looks awkward with dreadlocks and a spike sticking out of their chest. Don't hug Lucario.
So, both are pretty awesome in their own regard, but I'm voting Lucario because he's the OG.