I'm an Office fanboy, and have been using 365 since it came out. I also train corporates how to use it, so not surprising.
Big must is for advanced use of Excel. Nothing else comes close. Relational databases in Excel, Power Pivot, Power Query, BI, some of the new functions. Plus little things like Slicers and Flashfill
If anyone uses Tableau, then they shouldn't think twice about investing in Excel 365
Outlook: definite improvements, speed, searching etc
Word: nothing major, and obviously could use Word 95/97 to write War & Peace quite happily. Not Wordpad since the long document tools are a must - for navigation, for formatting, for organisation, cross referencing.
PowerPoint: lots of things to like if incorporating video (BIG improvements), the rest is largely cosmetic. Faster tools, but not requirements.
OneNote: awesome app. Nothing comes close for power and integration with the rest of Office.
Access: used to train people on building databases but not any more, so don't really care how it's improved (or not).
OneDrive: I use it all the time over a desktop, a laptop, a Surface Pro 3 and a Lumia 930. Love it. No integration or data update issues.
Rest of the family use the other 4 licences from the 365 subscription, so works for me.
I also use Google docs for WP & spreadsheet sharing, since most people seem to like it more than using Office online.