Originally Posted by Fat LucaroarkZ
I actually can't really see Riolu being any higher than Middle Tier, if not Low Tier. When it evolves, Lucario really suffers from having weak STAB until it learns Aura Sphere, which doesn't happen until the extremely late Level 51. Its level-up movepool certainly doesn't make up for it, since in between learning Force Palm and Aura Sphere, it learns useless moves like Quick Guard and Heal Pulse. Not to mention that when I had it, it was just so rarely useful. It's also really rare (5% encounter rate...). Fighting moves are super effective against no gyms except the first one, where Riolu is not likely to have learned Force Palm by then. It's not really useful in any notable battles except against Grimsley, Iris, and the Shadow Triad, all of which are late in the game.
Lucario does have poor STAB until quite late, but that hardly holds it back. Just upon evolution, it'll have a very strong Return to work with in addition to Force Palm. Which is in and of itself enough to take on Burgh if you have nothing better.
-Access to both Dig and Rock Tomb for Nimbasa, so it's got coverage on literally everything there.
-PWT, it plows through both Cheren and Corless.
-By Mistralton, it can use Rock Slide and its also perfectly reasonable to have Swords Dance around this point too, since you get the Lucky Egg. Swords Dance+Rockslide will wreck Skyla just as well as anything else (actually, Swords Dance lets you wreck whatever you want if you can outspeed and take a hit; Steel typing makes that happen most of the time, too). Even if you're out-sped, Steadfast versions have a pretty good chance for the boost, too.
-By Opelucid, Ice types aren't exactly widely available. Steel typing is very useful here and Lucario CAN learn Ice Punch if you cough up the shards.
-After Opelucid, it also wrecks any Dark or Ice type pokemon encountered with Team Plasma, too.
-While it does absolutely wreck Grimsley the most and handles Iris well, Lucario also has Shadow Claw for Caitlin and Shauntal.
That's not considering how useful it is outside of major battles. It's got Fighting/Rock coverage fairly early, and can switch over to literally perfect Fighting/Ghost coverage whenever you want with Shadow Claw with the offensive stats to take advantage of it. It's level up movepool is pretty lousy for a good bulk of the game, absolutely, but it won't be relying on that anyway.
You were also saying that the fact that it particularly shines against various late game bosses like Grimsley and Iris was a mitigated point for it just because of how late these battles are. It still gets credit for it.
I can't see it going lower than Mid. I definitely think it's High tier, though.