I have a strong predisposition to running 7.1 on the Classic II, as it is stable and relatively simple. System 6 (there is a version that works) will make it fly, but 7.1 is still somewhat nimble.
However, I also have a vague recollection that one of our old and most knowledgeable members from years and years back, equill, ran 7.5.3 on a Classic II. And the machine was used for years and years like that. That would, however, presumably require that one turn most all the 7.5 fluff off. For instance, AppleGuide would surely make a CII slow to a crawl, and you will take real RAM hits from loading QuickDraw GX, Quicktime, and the like, to no real benefit. You might, however, want to get good AppleScript and some nice touches like hierarchical menus and so on from a stripped down 7.5.3.
As I understand it, there is nothing to be gained from running 7.5.5 on any 68k unit over 7.5.3; 7.5.5 was mostly for PPC. Someone will no doubt correct me If I am wrong.
System Picker will allow you to have multiple System Folders, so you can boot from installations of Systems 6, 7.1 and 7.5 on the same startup disk.