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I'll never forget my first, Mac OS X Puma. IIRC, it was during a time when the whole cat thing was really a code name, rather than a marketing name. It opened my eyes to the beauty and power of the Mac.
 
It baffles me how anyone can choose the trash that is Lion. That is the worse release of OSX to date, no doubt. I love ML, but I still have a special place for SL. That stepped up the game big time.
 
Leopard from an 'upgrades' point of view. A new look, Quick Look (so useful - trying to use XP at school is murder from a productivity point of view), Time Machine, upgrades to Dashboard, iCal, Photo Booth and Safari 3.

Probably Snow Leopard from a stability point of view - everything seemed so responsive and just 'worked'.

Mavericks from a 'what I'd actually use now' point of view - some great features and is definitely a lot better than ML.

So yeah, define 'best' :p
 
1) Mac OSX 10.3 Panther. After the first versions, it was a very beautiful revolution, really sleek and snappy on my dual G4@867Mhz and on my iMac G3 tray loading 266Mhz comparing to 10.0-10.2
2) Mac OSX 10.6 Snow Leopard. The last "old school" Mac OS X, it gave a better feeling to me than Lion and ML.
3) Mac OSX 10.8 Mountain Lion. More stable than Lion, solid feeling, but something of undefined is missing. Maybe too "iOSified", but this is the new Apple way.
I have to say that, although i don't regret old times, i think that something from the feeling from iOS9 is still missing.
 
1) Mac OSX 10.3 Panther. After the first versions, it was a very beautiful revolution, really sleek and snappy on my dual G4@867Mhz and on my iMac G3 tray loading 266Mhz comparing to 10.0-10.2
2) Mac OSX 10.6 Snow Leopard. The last "old school" Mac OS X, it gave a better feeling to me than Lion and ML.
3) Mac OSX 10.8 Mountain Lion. More stable than Lion, solid feeling, but something of undefined is missing. Maybe too "iOSified", but this is the new Apple way.
I have to say that, although i don't regret old times, i think that something from the feeling from iOS9 is still missing.

OS 9 was OK but lets not take it to much further back than that 7 and 8 were not OK..

I still think Tiger was the best!
 
10.5 Leopard was the first OS X I used after my auntie let me use her MacBook for a few hours after she went bed (I was up all night on it!) haha, 10.7 Lion was the first OS X I bought, but quickly updated to 10.8 Mountain Lion which was awesome. Hoping to get a rMBP with Mavericks very soon.
 
Snow Leopard hands down.

It was one of the few points in time where Apple actually took sensible, engineering focused decisions, and released an OS that didn't look all that different to the one before, but had been optimized, stablilzed and improved under the hood instead of just adding more pointless shiny crud like they do these days.
 
Tiger had the best implementation of Spotlight IMO (and also the first). I remember thinking it felt really solid. I liked the aims of Snow Leopard a lot, and it is very solid; although, I vaguely remember there being a few issues with the first couple .x releases.

In terms of most improved, that has to go to 10.1. It made Mac OS X a much more useable, viable OS.
 
OS X Mavericks will be the best.
Tiger was great but barely has compatible apps anymore
Leopard is my current favorite because it has most compatible apps on ppc :)
 
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