I try to go with commanders that most people would think of because all but one of my commander decks are for fun.
Monocolor:
White: Megeta the Lion - Kill you without creatures.
Blue: Tromokratis - Voltron.
Black: Mikaeus, The Unhallowed - Competitive Combo.
Red: Marton Stromgald - Tokens!
Green: Seshiro the Anointed - Snake Tribal.
Guilds:
Azorius: Brago, King Eternal - ETB Blinking.
Dimir: Lazav, Dimir Mastermind - Copy all the things.
Rakdos: Olivia Voldaren - Vampires.
Gruul: Omnath, Locus of Rage - Ramp.
Selesnya: Captain Sisay - Legends!
Orzhov: Ayli, Eternal Pilgrim - Lifegain/Control.
Golgari: Glissa, the Traitor - Artifacts.
Simic: Edric, Spymaster of Trest - Stupid Things.
Izzet: Tibor and Lumia - Charisma, need I say more...
Boros: Brion Stoutarm - Throw big creatures at you.
Tricolor:
Bant: Roon of the Hidden Realm - More Blinking.
Esper: Sydri, Galvanic Genius - Artifacts
Grixis: Marchesa, the Black Rose - Fun Police.
Jund: Karrthus, Tyrant of Jund - Dragons!
Naya: Mayael, the Anima - Big Things.
Mardu: Kaalia of the Vast - Angels and Demons, and Dragons, oh my!
Temur: Riku of Two Reflections - 2 is better than one.
Abzan: Karador, Ghost Chieftain - Graveyard Play.
Jeskai: Narset, Enlightened Master - Storm!
Sultai: Damia, Sage of Stone - Ramp/Control
Other:
Colorless: Ulamog, the Ceaseless Hunger
WUBRG: Sliver Overlord - Slivers: all day, every day.