Tough choice,
for me, depends on what I'm shooting,
for general use, it'd be a toss up between an older Nikkor-O 35mm f2 or a nikkor-SC 55 1.2 (critics be damned, this lens is wonderful)
for portraits/pics of the grandkids, toss up between a nikkor 180 2.8 or a 135 f2.
for landscape type shooting either a 18mm, 20mm or 24mm (though my wife has 'tentatively' given me permission to buy a 15mm, and I think that will end up near the top of the list)
then there's the Canon 50 f.95 I had that was horrid, though I wish I had it now, after seeing what they've been fetching on the 'bay
erie