Good topic - and I like the qualifier.
I adopted a policy many years back of not selling old gear as long as I wasn't broke
so I have lots of my older gear - but they only come into play now and then.
So other than some old pedals and my guitars the oldest synth that I use constantly
is my Roland JD-990 and the oldest that I use semi-regularly
(1-4 times/year) are
my Moog Modular (an expanded/modified System 15) and Korg Monopoly. (The Korg has an
empty spot on the top left that's great for placing effects units for quick manipulation.)
They both get played out from time to time - see pix.
Moog at the top of the back pile/JD-990 at the bottom.
That's my girl Lynn Harrigan on the right reading from her book of poetry Moon Sea Crossing
Not a clear shot of the Korg Monopoly I'm playing but the photo has a good vibe.
That's Jim Field of spacenoiz and Rhea's Obsession on the left.