Linux

I've been using it pretty consistently for years, I achieve uptimes upwards of 60 days like this. The downside is that it takes kinda long to enter that state and there's a chance that it fails to enter it (though it doesn't crash, just goes to the screenlock), especially on my laptop - it refuses to go into hibernation unless I kill most webbrowser windows, especially stuff like YouTube.

And you do need a pretty large swap space; with those super long uptimes, even swap size = ram size isn't always enough, because sometimes the swap space is actually used as swap and not just as hibernation image storage.