big wheels
If I ever have to go 16, I'm sure I can find some that look good on each car, but until I'm forced, 15 is my limit. Bigger wheels can look good on older cars if matched well and if you're going for a more modern look, but I like the classic look.
That said, the 14" tires I have on my 72 are probably the last 14's I'll buy. They will go on the GTS, then once worn out or too old, I'll probably replace everything with 15's from then on. Just too hard to find decent and affordable 14" tires in a good size now. The bigger issue is SBP on the '68, but I'll deal with that one day if I need to.