For me, it's a tie between a bidet and a standing mixer. Both of those were utterly life-changing for me, and with both, I regretted not getting them years earlier.
I was nervous about a hard stream of water going up there, not knowing how cold or hot it would be, but it was a life changer.
First thing in at a new house for me is $50 bidet.
I'm rolling with Humanscale 6G keyboard tray and magnetic USB-C right-angle connectors.