Best OS for a C2D Mac Mini?

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I've got a C2D Mac Mini which I specced up to the fastest CPU at the time, 500Gb 7200RPM drive with 4Gb RAM.

I appreciate that it's low spec compared to current systems and I never took it past Snow Leopard.

I have no need for it any more so I'm going to sell it but I wonder what would be the best OS to put on it. I'm assuming it'll be a bit of a dog with Mavericks so should I go with Lion or Mountain Lion?

Bottom line - what's the best OS for my C2D Mac Mini?

 

[Feek]

Mine is a late 2009 model, the one just before it went all metal so it's a bit taller and white.

 

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I'm installing it now, thanks.

 

[Feek]

Yup, it's working well, thanks.