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