MacBook Air 1.86 GHz processor, 2 GM memory - what's the most recent OS I can upgrade to that will still run efficiently?

Hello,

I've got an older MacBook Air (bought in approx 2011).

Model Identifier MacBook Air 3,2

Processor 1.86 GHz Intel Core 2 Duo

Memory 2 GB 1067 MHz DDR3

Harddrive 120 GB

I'm currently running Mac OS X 10.6.8

I'd been told a couple years ago by someone at the Genius Bar that I should not update to the newly released OS back then (can't recall which version it was) as it would be too cumbersome for me to use on this older machine.

Now I'm having issues where Safari and other applications are not able to update any further unless I install a more recent OS. This morning my Outlook will no longer function... big problem for me at work.

Can anyone tell me what the best & more recent OS I can confidently install that will not cause my system to slow down considerably?

Appreciate the help!