My favourite sweeper is LO Dusknoir. With 3 attacks + Pain Split it really combats alot of pokemon despite it's mediocre speed. My Favourite wall by far has to be Tangrowth. It wall all physical attacks like a pro. Taking ~50% from CB Scizors U-turn and Infernape's Flare Blitz.
Porygon-Z is my favorite lead/ sweeper. It's a personal preference though- his line has been my favorite since gen1. Slap a scarf on him and he's still an awesome starter pokemon.
Sweeper: OU: Offence Cune UU: Mono attack Herra NU: CM Corsola ♥ Walls: OU: Crovile (But srsly Gliscor) UU: Milotic NU: Dusclops/Lickilicky
Of all Pokémon that can serve as a sweeper, my favorite is Dragonite As for walls, I never use them as they hardly suit my battling style. However, of all Pokémon that can serve as walls, my favorite is Lugia.
Sweeper = Agilitycross :D Metagross with the +2 Speed Boost is epicness! Wall = Blissey, Not your average Sp.Def Wall and obviously not your choice for a Physical Defense wall. But how many Special Sweepers can tear it apart?
My favorite sweeper is Absol, but it's hard to pull off. I don't have a favorite wall, but a favorite tank would be Gyarados.