Well any Pokemon can be a good let game sweeper if its is able to get the boost from other Pokemon as this guy has proven many times by doing his "Epic Sweep Videos", but there are some that stand out more than others.
Ubers-Mewtwo is the first one I can think of, because once someone loses their Mewtwo counters this Pokemon can become fatal and win you the match.
OU-Volcorona is of course one of my favorites, once you allow it to set up 2-3 Quiver Dances you team is likely to be swept. Genesect also works when scarfed or expert belted, just take out all the Pokemon that outspeed.
BL-I wouldnt really consider this a tier but if you want it ill put it here,Gothitelle can do damage if you let it shadow tag you into a not-effective move and it sets up Calm Mind.
UU-Choice Scarf Moxie Heracross will always clean up the game at the end.
BL2-Again same as BL but ill include it,Durant is the one that comes to mind for me.
RU-Unburden Secptile or Unaware Quaqsire make great sweepers at the end of the game bearing of course you get the speed boost with Secptile and you set up with Quaqsire and the opponent has no chance to hit you hard seeing as you ignore their stat changes.Feraligatr with Sheer Force can also do damage.
NU-Ninjask along with Shell Smash Gorebyss can clean up the NU metagame.Gurdurr is also an option.
LC-Choice Banded Adaptability Eevee with Quick Attack can do damage once all the opponents Pokemon with resistance or immunity to Quick Attack is gone.Abra also works with its Speed and Special Attack.
Limbo-Only one Pokemon there, so yea.
NFE-Moxie Krokorok works well at the end of the game.