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.
"If you put the federal government in charge of the Sahara Desert, in 5 years there'd be a shortage of sand." - Milton Friedman
I am not lucaroark on Pokemon Showdown.
I'm always on PS as Adamant Zoroark.