The one that would least upset me would have to be my DA*55 (that new 50mm would be a welcome one to be glued it though...
)! Surprisingly, the DA* has left my 77mm a bit in the dust - I should get myself reacquainted with it, as I do like the longer focal length better. DA*200mm would be be next after the 77mm.
Are APS-C lenses involved in this? I have only used primes on the K-1, but I suppose the 18-55mm that I haven't used in years would make me sad. If just full frame lenses, probably the F 28mm 2.8. A fine lens for when I need it, but not one that suits my style of photography.