Another possiblity to consider
is Linux. Ubuntu appears to be the most common flavor of Linux these days.
I've been using Linux as my modern OS since 2005, and have (mostly) been happy with it. There are some advantages Apple has over Linux--some software choices, particularly for graphics work. However, Linux is good enough for most "normal" uses. I certainly haven't felt limited--but then my major need for a modern OS is the Internet. If it weren't for the Internet, I could get by easily with a 90s computer.
Linux can run on basic Windows hardware. However, not all hardware is 100% compatible--some computers (particularly laptops) can have issues.
Installing Linux is also an issue for many. This has gotten easier over the years, and I'd consider it easier to do than Windows. But installation hassles are one thing a lot of people hate. Then, again, many people never install an OS--they buy something at Office Depot with Windows installed, and when it goes bad they replace the system, or have someone fix it.
Once installed, though, Linux has always run reliably for me.