What YC company would you consider innovative and groundbreaking - at the stage when they were small enough to be in YC? Even the massive successes were still

What YC company would you consider innovative and groundbreaking - at the stage when they were small enough to be in YC? Even the massive successes were still essentially novel takes on existing ideas, when they were in YC: Airbnb as Couchsurfing variant; Dropbox as file sharing service with a better UI; Stripe as PayPal but not terrible. Etc. I wouldn’t really expect the average company at this stage to be groundbreaking.