I would say Citi Double Cash, accompanied by a rotating categories card like Discover It or Chase Freedom, and/or a special categories (dining, groceries etc.) card. That way you are always earning at least 2% and often more. But if only one, I’d go for Double Cash.
But it’s been years since I applied, maybe there is a reliably 3% card now?
Edited to add: my suggestion above assumes no annual fee for all cards. If you are out to maximize cash back, I think an annual fee card is usually the wrong choice.