I apologize for saying this in advance but I'd have to say for me it's GIMP.
Even after all its feature and popularity I find it extremely hard to use and often end up wasting hours trying to create a simple graphic. As a last resort I always end up booting Windows just to use Photoshop to do the same. I have been trying to move away from Windows to linux and so far I'm super happy with everything. But Photoshop is the only reason I have to keep Windows alive (or atleast use wine)
There was recently a post on HN about a GIMP fork called glimpse which aims to improve UI/UX: https://github.com/glimpse-editor/Glimpse or project page at https://glimpse-editor.org/