Which sync method should I use?

iCloud is the recommended sync method for 2Do on iPhone, iPad, and Mac, and on Android too when every device runs a version of 2Do with iCloud sync. On Apple devices, everything syncs through the iCloud account already signed in on the device, with no separate account or password. On Android, you sign in once with the same Apple Account. iCloud carries every task property including List Groups, Tag Groups, Smart Lists, and attachments. If a device runs an older version of 2Do without iCloud sync, or you don't use an Apple Account, 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. Manual order in the Focus Lists syncs only with iCloud.

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

• iCloud: recommended, with complete feature support. There is nothing to sign in to on Apple devices; on Android, you sign in once with your Apple Account. See iCloud sync (Mac), iCloud sync (iOS), or iCloud sync (Android).
• Dropbox: the choice when a device runs an older version of 2Do without iCloud sync, or when you don't use an Apple Account. It carries task fields, groups, Smart Lists, pictures, and audio notes; link-only attachments stay on the device. See Dropbox sync (Mac), Dropbox sync (iOS), or Dropbox sync (Android).
• 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), Toodledo sync (iOS), or Toodledo sync (Android).
• iCloud CalDAV: Apple-hosted CalDAV with your Apple Account and an app-specific password, worth choosing when other CalDAV apps read the same lists. List Groups, Tag Groups, and Smart Lists stay on the device. See iCloud CalDAV (Mac), iCloud CalDAV (iOS), or iCloud CalDAV (Android).
• Custom CalDAV: Fruux, Yahoo, Nextcloud, ownCloud, Synology, Fastmail, and other CalDAV servers. Same limits as iCloud CalDAV. See Custom CalDAV (Mac), Custom CalDAV (iOS), or Custom CalDAV (Android).
• 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 devices, including Android. Choose Reminders if you want your tasks to live inside Apple's Reminders app; it is not available on Android. 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).