Python has bindings for pretty much everything, so I'd imagine that the database you're most familiar with, or that there is the most information on (most likely MySQL, Postgres, or SQLite)
I'd suggest SQLite unless you had a ton of users interacting all at once (if it's something involving editing excel spreadsheets, I doubt you'd have a problem with this).
Slightly off topic, but has anyone built an Excel-esque program on top of SQL? I feel like something of the sort could really be an improvement, as now it'd be easy for end users (who normally use excel) and developers to collaborate on the same data.
