I recommend any of the impressive tools that round-trips text nicely (Sublime, Notepad2, gedit). Just plain text with minor format annotations.
It's easy to lay out simple graphs which I use for financial summaries that I will be able to access for years.
Text is a massively-entrenched standard format with universal comprehension rules (unlike other standards like HTML). Just be sure your editor makes sense of either line ending. You can use many tools including those that can write directly to a RAID0 device, an FTP server, etc for safety.
Integration has some positive factors to consider:
- Copy-paste of text into a calendar works great.
- Filesystem tree layout is handy for archival purposes (and is also deeply entrenched).
- Naming in year-month-day-subject.txt consistently will allow filesystem search to find the note you want, and dates are retained despite the copy tool used.