For me it's Hotline Bling easy. The play count i had for that song on my Google Play is record breaking. 0-100/The Catch Up is a very close 2nd. There's too many to name breh. He dropped mad songs every summer that didn't make his album's. It's weird though because when i reminisce about it. It seems like one long ass Drake dominated summer. For example (without Googling). I know IYRTITL (probably my favorite Drake project) came out early 2015. But i be feeling like he dropped Back to Back, Hotline Bling, Versace Remix, Right Hand, We Made It, How Bout Now (IYRTITL album cut) etc that same summer.
Of course Summer Sixteen was the next year. My Way Remix might have been to.
*4PM in Calabasas, and All Me is slept on.