For me, the worst version was definitely 10.9 (Mavericks), with 10.7 (Lion) and 10.5 (Leopard) following closely. Both 10.7 and 10.5 were buggy, had odd glitches not found in their respective 10.8 and 10.6 counterparts. I never liked Mavericks from the beginning, though. The first developer beta left a bad taste in my mouth, and the OS just felt overall very chunky, buggy, and "unwrapped" (if that makes any sense). Release up to 10.9.5 had unfinished bugs in Time Machine (jumping back to the ~ folder when entering TM, oy! Many other odd bugs and sluggishness. Thankfully, solved in Yosemite for the time being), constant xpcd errors (also, so far, resolved in Yosemite), WiFi connectivity issues (by far fixed in Yosemite), and just general sluggish performance. Yosemite reminds me of Mountain Lion, coming from Lion, which was almost as bad as Mavericks. I find it odd and baffling that so many people laud Mavericks as the "next" Snow Leopard, or one of the best releases, while hating on Yosemite. No way, that's for sure. Mavericks was the worst release of Mac OS X. I mean, this may seem silly and nitpicky, but when I did a test upgrade from SL to Mavericks with multiple user accounts, all desktop wallpapers of each user account were deleted during the upgrade and reset to the default (ugly) Mavericks wallpaper, and there were odd glitches! Something as simple as that couldn't even be preserved during the Mavericks install. That's how buggy it was. The Migration Assistant also had odd lockups and freezes, at multiple points freezing the whole computer and resulting in a hard reset. Conversely, with Yosemite, when doing the exact same test upgrade from SL, all wallpapers were preserved beautifully, without any odd glitches (in fact, the computer actually felt more responsive), and the Migration Assistant behaves better for the most part. It's a well-known legend that Apple assigns the same programmers/engineers to work on every other version of Mac OS X. Meaning, the same engineers who worked on 10.6 (SL) were also assigned to 10.8 (ML) and subsequently, 10.10 (Yosemite). That would explain why odd-numbered releases of OS X has always been a bit more buggy/glitchy compared to the even-numbered versions. Does anyone else notice this or feel the same way about Mavericks? What is your overall opinion on the worst OS? My favorite is Yosemite. And, yes, this is on a non-retina MBP.