i have an old Macbook pro (mid 2012), running El Capitan, version 10.11.6. can i upgrade

Click on this link to read how to get different versions of macOS --> https://support.apple.com/HT211683

- The article also has links to system requirements for each OS version which you should check against your exact computer model. Note some of the model identifier articles are outdated. For any model produced in the last 5 years or so you should use the system requirements links for each OS version instead. This web site also has a reliable presentation of model and OS version compatibility: https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

- It helps to have more RAM (memory) than what is noted as "minimum" for any particular system or your computer may run slowly.

- You may find you need to use Safari to get links for High Sierra and later using the Mac App Store application to open a functioning page.

- Sierra, El Capitan, and Yosemite download as a .pkg. Click on this to have a converted version placed in your Applications folder as an "Install OS..." application. Run the installer and follow the directions.

- Upgrades may take some time (up to an hour or more) to install, with restarts and blank screens.

Before upgrading, back up at least your data in case something goes wrong. See this support document: How to back up your Mac - https://support.apple.com/mac-backup

Check if your old apps will still work with with the OS you want to install - https://roaringapps.com/apps This is particularly critical if you are upgrading to Catalina or newer, as older 32 bit apps will no longer work. To check for those, try this 32 bit application scanner application --> https://www.stclairsoft.com/Go64/

"Best" is really dependent upon your equipment, software, and personal preferences. Obviously Apple thinks the newest is the best. With 10.11 you may have a lot of old apps on your computer. Eventually those will need updating but if you go to Catalina and suddenly find you have to spend $700 in software upgrades then you may decide to take it in smaller steps.

If your computer does not have at least 8GB RAM then you may also not want to be running some of the newer memory-greedy versions. Also realize that just after your Mac was produced, solid state drives became common in Macs and the newer systems kind of assume you have one. They work on conventional HDD but I think with either Mojave or Catalina it will restructure the disk to APFS which is kind of hard on older HDDs. For both RAM and drives your older computer can probably be upgraded for not too much but that is up to you, particularly if you are not a do-it-yourself person which will drive up the cost.

I'll let others provide feedback too.