Libre Office, without a doubt. Setting aside any discussion of libre-licensed vs. proprietary software, MS Word and Excel used to be my favorites, both before there was such a thing as "Microsoft Office" and up through Office 2007. However, I had Office 365 running on this very computer under Windows 10, and honestly I thought it absolutely sucked. I don't mean the visual aspect of the UI — once you've gotten used to Office 2007, the subsequent releases are all more or less along the same lines — but rather the performance. It is trying so hard to be "silky smooth" but it just comes off as sluggish and like it's running through some sort of emulator or cheesy web interface. I mean, I've used those sorts of things before, particular Google's cloud-based office suite, and even it is far snappier and more responsive than running the local copy of O365.
Sorry, I didn't mean to head off on a rant about Microsoft's products. As I said, they used to be so solid and pleasant to use. But, for my money, LO Writer and Calc work better and more reliably than anything else out there right now.