Since it's been a while since this question has been answered, and this machine still seems to be in use (at least by the question author and myself) as of last year, I'll toss in a three-part answer from 2023's perspective:
- The ideal macOS for this machine is High Sierra, the last official macOS available for this machine from Apple. Unofficially, this machine can run newer versions of macOS. A bit of online searching for "unibody MacBook 2009" along with the desired macOS in question can yield some interesting results.
- Mountain Lion was the preferred version of "OS X" over Lion back in the day, as El Capitan was to Yosemite and High Sierra was to Sierra. I doubt you care to go back in time to Lion anymore. I certainly wouldn't recommend it :-)
- As you've found (from your comments), upgrading your memory was a great and very simple approach to improving this machine's performance. I've had mine up to 8GB but it's back to 6GB now as I don't use the machine all that often. One can still get by with as little as 4GB on a modern machine as long as they have also upgraded it to an SSD (as you have since asking the question).
In summary, upgrading this machine's memory to 4GB or higher and the hard drive to a SATA SSD still provides quite a bit of horsepower for such an old machine, as Apple was fairly generous with the hardware specs for the Late 2009 White Unibody MacBook machine in comparison with the MacBook Pros that were available at the time.
You may also find that a replacement battery is in order, depending on how much life you are still getting from your original battery. I am on my second battery in 2023, and I generally baby my batteries by running the laptop on AC power most of the time, while letting the batteries drain out every several weeks.