I don’t mean the obvious “well, the bike” answer, more the piece of kit that’s been a constant on your riding adventures for some time; the piece of kit you reach for because it just works.
For example: mine is probably my Wingnut MPS pack. It made me smart a little when I bought it back in 2008, but has been used in various guises at least once a week since, and modded to add little functional features I wanted. It’s been used all over Europe and been up and down more peaks than I care to count. It’s tough, incredibly comfortable and intuitive; I’ll be genuinely upset when it finally falls apart.
Is there a piece of kit you’re genuinely attached to? Something that ‘just works’ for you?