Ditto on Snow Leopard. I had to muck through the version history to figure out which release I was considering the "last good, exciting release." Turns out we all agree. The divide is... stark. That said, High Sierra seems to trend back up towards the level of ambition we saw in the 200x releases.

    2001 10.1 Puma:          First OS X to ship by default
    2002 10.2 Jaguar:        GPU compositing, Mail.app, Address Book, MP4
    2003 10.3 Panther:       Safari, iChat AV, Journaled FS, Apple's PDF engine
    2005 10.4 Tiger:         Spotlight, Dashboard, Automator, Core Image/Video
    2007 10.5 Leopard:       Core Animation, Time Machine, Boot Camp, 64 bit, intel
    2009 10.6 Snow Leopard:  App Store, XCode overhaul with LLVM
    2011 10.7 Lion:          Launchpad, auto-save documents, multi-touch
    2012 10.8 Mountain Lion: Game Center, Notification Center
    2013 10.9 Mavericks:     iBooks, Apple Maps, iCloud integration
    2014 10.10 Yosemite:     Skeuomorphism -> Fisher Price, Continuity & handoff
    2015 10.11 El Capitan:   San Francisco font, Metal API
    2016 10.12 Sierra:       Siri, Auto Unlock, Night Shift
    2017 10.13 High Sierra:  HEVC, APFS, VR