Ooooh... For me at least, it's a tie between three items. The Bolder LC40, the Powercore 26800, and the Powerline II.
Bolder LC40- In case you don't know, this flashlight is INSANELY bright, and can practically light up a room. In power outages, this comes in use quite a bit, as well as regular dog walks at night.
Powercore 26800- I just got this from Prime Day, so I haven't had much time with it yet (review coming soon). However, I have been able to charge an iPhone 8 plus, iPad, and a kindle all night for at least a week, without it dropping below 2 "bars" of remaining power. It's really cool!
Finally, the Powerline lightning II. I went through countless lightning cables before getting this one. It is incredible.
Oh, and the Soundcore 2, SoundBuds Slim, PowerPort Wireless, and the PowerPort Speed 4 are all other daily drivers.