E36 M3s, E39 M5s and R32 GTRs definitely aren't going to get any cheaper so a clean one of any of those would get my vote. I have been keeping half an eye out for a good one at a bargain price but it would mean selling my Z, E28 and then some so wouldn't be an easy swap.
Think only the E36 would be a realistic daily too. M5s I read struggle to get 15mpg if you are even a bit heavy footed and no doubt and ageing RB26 is bound to get expensive and troublesome sooner or later.