what mac os i should download

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence.

macOS Catalina 10.15 hardware requirements

macOS Mojave 10.14 hardware requirements

macOS High Sierra 10.13 hardware requirements

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal  operation of the OS and may even Corrupt the OS requiring a Reinstallation

That would be your choice. I'm using a 2010 MacBook with a mechanical hard drive and 8 GB of memory. I'm running Mac OS High Sierra and it runs just fine.I did have Yosemite. I see no difference in speed from ether. I cannot upgrade any father, you can. Again to increase speed or performance you would need to upgrade your memory and hard drive.

Upgrading your Mac is always a good idea. Speeding up your computer would involve updating the memory and going from a mechanical hard drive to an SSD. What operating system are you running?https://www.macworld.co.uk/feature/what-version-macos-compatible-3776008/ This should tell you what you can update to. Then you can go here to find the date you want or can use. https://support.apple.com/en-us/HT211683 If your running Sierra, you might want to start with High Sierra before moving on to something newer.

OK. If you are happy with that, good. You can always upgrade further if you decide to. As more upgrades come out from Apple their support for older operating systems drop off.