A near mint Tamron SP 300mm f2.8 (360B) Adaptall lens. I've had my eye on this uncommon lens with its fast aperture, ED lens elements, and internal focusing. I've always wanted a superfast very long telephoto, and this one fits the bill, being the last and best manual focus 300mm f2.8 lens made by Tamron, and fast internal focusing. Of course, this thing weighs over 5 lbs!
I already have an OM Zuiko MC 300mm f4.5, which takes excellent pics and is pretty compact for a 300mm, but I don't like that the focusing ring is very heavily dampened (although very smooth) on this lens model, which makes it somewhat difficult to use for sports or wildlife photos as it is slow focusing.