I'm going with System 7.

My reasoning: it's got a nicer overall appearance on color systems than its predecessor, runs just about anything from the late 80s to early-mid 90s, doesn't use a ton of RAM or hard drive space in its earlier versions, can run from a floppy (barely, but it still does), and provides the classic Mac experience with some modern conveniences.

System 6 is a close second, followed by a tie between 8 and 9, then the 1-5 versions (too old to really be usable), and OS X is dead last on my list.