What is the best OS for my 2009 Macbook Pro?

@tinchoval, This thread might helpful to you upgrading os x to EI capitan, please go through this below links,

- OS X Snow Leopard - Apple Support

- Upgrade to OS X El Capitan - Apple Support

A mid 2009 MBP will accept up to 8GB RAM. How much do you have installed? Yosemite is a RAM glutton and additional RAM may very well speed up your MBP.

Ciao.

If you want the most efficient OSX, it will be Snow Leopard. Each successive OSX has been more bloated than the one before requiring more RAM to run efficiently.

In my opinion, go back to Snow Leopard or upgrade your MBP so that it can run Yosemite properly, hence my prior suggestion. If the bank account permits, an SSD would also be a step in the right direction.

You also may have third party software that is slowing your MBP such as an AV application or some so called performance or cleaning application.

Installed in a proper environment, Yosemite should be able to run very satisfactorily.

Ciao.

Your MBP can run 10.10 quite well but not with 2gb of RAM. If you cannot upgrade the RAM or don't choose to, 10.6 is the right OS. Even lion (10.7) won't run well with 2GB. For Yosemite to run acceptable on your Mac you'll need 8GB or 4gb and an SSD. Best performance would come with 8GB and the SSD. I speak from experience. That was the migration I made with my 2009 MBP.

Don't bother upgrading this computer's memory. the issue you're experiencing is due to the OS upgrade! I was told by a genius at Hillsdale, that this 2008 MacBook Pro was so slow after he upgraded to Yosemite... I'm having issues running el capitan, but its likely that the best version of the OS to run is either 10.7 (soon to be obsolete) or 10.8... I'm in the process of getting mine checked at the moment

I am running El Capitan on my 2009 MBP with 8gb Ram and a Momentus XT hybrid HDD 500GB and mine works fine.

How did you intalled the El Capitan on your 2009 mac? I am trying but it says "there are no updates available for your mac"