MacOS 8, released in 1997, is commonly considered the revision that make people sit up and take note -- the most visible OS Change since MacOS 3.
Notice I said visible. This is because although MacOS 8 looked completely different, a very small portion of the operating code actually changed from the previous major release, 7.5.5-7.6.1.
The visual changes were, to be fair, monumental. For years prior, the MacOS has been derieded has having an 'old' look compared to it's archrival Microsoft Windows which had been using 3d buttons since 1993 and size-adjusting scrollbars since 1995. MacOS 8 added the buttons, new icons and all the pretty colours that finally made the OS look modern again.
Other things it included was Personal Web Sharing (a pocket-sized web server), inclusion of MacOS Runtime for Java and a PowerPC Native Finder (though a 68x00 version was still available).
MacOS requires at least a 68040 based machine with at least 12 megabytes of RAM, though it runs just swimmingly on PowerPC based machines, too.