Most suitable Mac OS 10.6 through 10.9 for early 2009 iMac
I would suggest to use Mac OS X 10.6.

For my iMac 24" I have tested all newer versions (on different volumes), and 10.6 is running as it should be. 10.9 seems to be okay, but sometimes I see the "ball" and it takes more time until I can go on...

But it depends what you want to do and what you like - newer systems offers new features (and for me this is exactly why I stay on 10.6). AutoSave and recent application (documents) after computer (re)start might be keywords...

By the way: I would recommend to install LittleSnitch. This tool enhances the firewall for outgoing messages.