I agree with Mid-Tier for Riolu-line. It starts as a killing machine after it has learned Force Palm but will struggle against most gym leaders and trainers for most of the game. Counter was pretty nice when it worked out but usually I found myself being forced to switch Lucario out because of Fire and Ground pokémon/moves which seems to be everywhere in BW2.
My Lucario started to shine against E4 when it got a chance for couple of Work Up + CC and it Quick Attacked Iris' Haxorus out. <3
Axew-line for High Tier/Top Tier. Do I need to explain? DD + Dragon Claw/Shadow Claw OHKOs everything and you can solo rest of the game if you feel like so. The only flaw is that you have to wait until Cheren gives you Surf TM to have a chance to catch one of these cute killing machines. Steel-types may cause some pain too. :p Fraxure evolves into Haxorus at lvl 48 which is right before last gym so you you will sweep rest of the Team Plasma, transformed Kyurem and E4 like nothing.