Joe -
for me there is no question at all: it would be the 18 Par 4s.
First, because Par 4s are "golf" in its essence, i.e. a long shot, and approach shot, a recovery/lag putt, and a short putt, all in one neat package.
Second, because in being the essence of golf, I think architects instinctively know how to design Par 4s well -- and understand how to take advantage of/leverage the "main variable" amongst golfers of different ages and skill levels (i.e. distance off the tee) so as to make such holes engaging/challenging for a range of golfers.
Third, because Par 5s -- especially excellent Par 5s -- are much harder to find/create, not least because of the "multiple variables" inherent in holes of such length; and
Fourth, because a course with 18 Par 4s also gives me, potentially, the experience of 18 Par 3s at the same time; on each Par 4 I have the chance to have a short/mid/long iron in my hand (for my second shot) and to take dead aim at the green/pin, which is what Par 3s are all about.
In short, with an all Par 4 lineup, I get all the good bits about golf (and quality gca) with none of the potentially boring bits (e.g. the 2nd shot "layup" of the Par 5s)
Peter
Peter