Every year Apple releases a new version of OS X and your machine meets the requirements to run it. But will it run properly and smooth or will you be sitting there watching the progress bar waiting for it to fully boot?
Some of you might even upgraded the memory and to a SSD (Solid State Drive) and its still not running as expected.
Most OS X are designed by Apple to run smoothly on the most current systems. They look great, most of the time have new features you could use but at the end of the day its not a given that they will run the same way on your MacBook Pro / Air, iMac, Mac Mini or Mac Pro.
Over the past 2 years I have been playing around and documenting which OS X is most suitable for your Mac.
2008 MacBook Core 2 Duo (white or black MacBook) and earlier with the 32bit structure, just stick with OSX 10.6/10.6.8 Snow Leopard. Yes, you can upgrade to OS 10.7 but the battery consumption and overall performance will suffer. 10.7 Lion is the end of the road.
The same affects the same years Mac Mini, Original/2008 and 2009 MacBook Air and 2007 Mac Pro
2009/2010 MacBook white and Unibody: 10.8.5 Mountain Lion. They can be upgraded to the most recent OS X, but will run sluggish and slow.
2008 - 2010 MacBook Unibody / MacBook Pro Core 2 Duo and Air: OS X 10.8 Mountain Lion
2010/2011 MacBook Pro / Air i5 and i7: OS X 10.9 Mavericks
2012 and newer: Run whatever you want, but trust me when I say that Yosemite is not always the best choice. My suggestion would be Mavericks, again.
I had quite a few customers already, who did not like how their laptop or desktop performed and ended up downgrading to a older version of OS X. In some cases the most recent recent OS even created overheating issues. In some instances I even showed boot and performance differences on similar machines.
Example: 2010 MacBook Pro 2.4Ghz C2D with no upgrades 10.8.5 VS 2011 MacBook Pro 2.4Ghz i5 8gb Ram and and SSD. The 2010 MBP booted 3 times faster!