Comment by worldvoyageur

These guys [https://www.scientificballoonsolutions.com/news/], as of October 2018, claim the world record. [edit: the site linked says Alan Adamson but an upstream post mentions Lee Meadows. That thread also credits Bill Brown as a pioneer - https://www.stratoballooning.org/membership#!biz/id/5f4d7b97...]

The world record balloon was launched on September 21, 2016, stayed up over 767 days, circled the world 35 times and traveled over 1.5 million kilometers.

Their website, still from 2018, indicates they are working on even better designs.