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


For me, it's my Zerostat. Bought it in the '80s- most likely 1986 or 1987. 
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My LP's. Then my Revox A77 Mk3, bought new in '73 (from Walter Davies, then the owner/operator of a 1-man hi-fi shop in Livermore, California, more lately the developer of the Last Factory line of LP/tape/CD care products). 
While not the original two pairs of DCM Time Windows I had back in 1977, I am using a pair of late-production TW -1A's. Again, found on eBay in excellent condition!
MIT interconnects from the mid 90s and I think my PS audio extreme speakers cables too.