As with everything - it depends. You're asking me so these are *my* opinions - and I do not claim to be anything more than a hobbyist/tinkerer when the comes to this.
mbox has potential use for long term read-only archives - I see no reason to use it for live mailboxes.
maildir is undoubtedly the least susceptible to corruption. It's also the slowest format for reading. How slow is "slow" depends on your hardware - it may be imperceptible with enough RAM and SSD's - or it may result in user complaints with large mailboxes.
dbox is Dovecot's preferred format. I know Timo has put a lot of effort into it. sdbox is similar to maildir in that each mail is a separate file. mdbox significantly reduces the number of files which can make file-based backups faster. Both dbox formats are dependent on their index files.
If you've got good hardware, including a proper UPS, I'd recommend dbox (my server is presently using sdbox). With large mailboxes and file-based backups you'll benefit from mdbox. When reliability is the #1 concern above anything else - use maildir. Depending on your use SIS can have significant impact on storage requirements - but storage these days is relatively cheap.
I haven't seen much feedback from users actively using SIS - I'd love to hear from high traffic sites with SIS experience to know if the corruption issues have been resolved. In my case there was at least a 30% reduction in space but I had too many errors - admittedly it's been a couple years since I last tried it.