Redditors at /r/churning are known to freely share the greatest credit card sign-up bonuses, and the latest seems to be a 60,000 SkyMiles for the Delta Platinum or 50,000 SkyMiles for the Delta Gold, both offered by American Express.
Even better, the Delta Gold card comes with a $50 statement credit, and the Delta Platinum a $100 statement credit, after you make a Delta purchase within 3 months of card opening. In the past, qualifying purchases have included everything from airfare to drinks, eCerts to on-board entertainment (but not WiFi).
The Delta Platinum offer is particularly appealing because it comes with a 10,000 MQM offer, in addition to the 60,000 RDM miles, for a relatively low spending requirement of $2,000 within the first 3 months.
You can find what appears to be a public offer at this special American Express landing page.
Both offers expire on July 6, 2016 so I wouldn’t sit around too long if you’re thinking of applying. Correct me if I’m wrong, but I believe this has been one of the best (the best?) publicly-available offers for either card.