Who wins?
Saying "Uno" vs Challenging a Draw 4
Saying "Uno" wins 54% to 46%. 113 more put them in the same row. From Board Game Rule Debates Tier List. Pick a side.
RandomPossum on Saying "Uno": it's printed in the rules, two card penalty if you forget
OldPigeon369 on Challenging a Draw 4: actually in the rules, show the hand if challenged
Board Game Rule Debates Tier List: the full ranking