What, in your system, have you owned the longest?

Tandberg TD20A, since 1979.

Ha! 1979! Probably still has that new component smell!

Mine’s a Pioneer TX-9100 Tuner, purchased new from the Tacoma Jafco with paper route money in 1974.

This however depends on the exact meaning of "in your system" since if you look at my system, which I encourage at all times and whenever possible, you will see way down low over on the right said Pioneer Tuner. https://systems.audiogon.com/systems/8367 No not the very bottom, that would be the Technics SL-1700 which is nowhere near as old, having been bought in 1976.

In any case while both of these are in my system room and in my system pics they are not at the moment actually connected. And so you being the OP and with of course having the hottest avatar I submit to you My Liege for this ruling.

If you rule against the Pioneer then that will of course apply to the Technics leaving the humble, vintage 1976 ZeroStat, burnished well with a patina of fine scratches yet still streaming ions like the day it was new.

My amp acquired in 2004 it's a custom stereo tube amp from Deja Vu Audio in VA.

Purchased in 78 or 79.  JBL 65. and McIntosh MA 6100 from Moe’s Sight and Sound, Casper, Wyoming.  MA 6100 being refurbished.

SOTA Sapphire.  Early 1980's.  Suitably upgraded over the Centuries.