Which sync method should I use?

iCloud is the recommended sync method for 2Do on Apple devices. Everything syncs through the iCloud account already signed in on the device, with no separate account or password, and it carries every task property including List Groups, Tag Groups, Smart Lists, and attachments. If you also run 2Do on Android, choose Dropbox, which carries the same task fields and groups through a folder you control. Dropbox syncs picture and audio attachments but leaves link-only attachments on the device.

2Do syncs through one account at a time, and every device should use the same method and account. The methods are:

• iCloud: recommended, complete feature support and nothing to sign in to. See iCloud sync (Mac) or iCloud sync (iOS).
• Dropbox: the choice when Android is involved. It carries task fields, groups, Smart Lists, pictures, and audio notes; link-only attachments stay on the device. See Dropbox sync (Mac) or Dropbox sync (iOS).
• Apple Reminders: native sync, so your 2Do tasks also appear inside Apple's Reminders app. Attachments, standalone locations, and 2Do's grouping features stay on the device. See Reminders sync (Mac) or Reminders sync (iOS).
• Todoist: your Todoist projects appear as 2Do lists. Attachments and standalone locations stay on the device. See Todoist sync (Mac) or Todoist sync (iOS).
• Toodledo: useful if you already use Toodledo's web client; Actions and attachments do not map. See Toodledo sync (Mac) or Toodledo sync (iOS).
• iCloud CalDAV: Apple-hosted CalDAV with your Apple Account and an app-specific password, worth choosing when non-Apple devices read the same lists. List Groups, Tag Groups, and Smart Lists stay on the device. See iCloud CalDAV (Mac) or iCloud CalDAV (iOS).
• Custom CalDAV: Fruux, Yahoo, Nextcloud, ownCloud, Synology, Fastmail, and other CalDAV servers. Same limits as iCloud CalDAV. See Custom CalDAV (Mac) or Custom CalDAV (iOS).
• WebDAV (coming soon): will sync everything to your own or any WebDAV server, with nothing left behind. See WebDAV sync (Mac).

The three Apple methods are different from each other. Choose iCloud for the most complete sync between your Apple devices. Choose Reminders if you want your tasks to live inside Apple's Reminders app. Choose iCloud CalDAV for Apple-hosted CalDAV task lists that other CalDAV apps can read. You can only run one of them at a time.

For a full per-method comparison of what each one carries, see Sync overview (Mac), Sync overview (iOS), or Sync overview (Android).