my os is yosemite what os is best for upgrades ?

The model year of an iMac is available at Apple menu > About This Mac.

An early 2009 iMac can be upgraded to El Capitan 10.11.6 but no later version. See: How to download macOS - Apple Support

A late 2009 through mid 2011 iMac can be upgraded to High Sierra 10.13.6 but no later version. See: How to download macOS - Apple Support

Back up your data, check your applications for compatibility, click here, and depending on which iMac model you have, then on the download link for El Capitan or High Sierra.

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