You can sync your contacts, emails and calendars from the HTC One M8 for Windows to the iPhone 6 or 6 Plus using a Google account:

Setting Up a Google Account on the M8 for Windows:

1. From the App list, tap "Settings"
2. Go to "Email + accounts"
3. Tap "Add an account" > "Google"
4. Type in your email address and password and tap "Sign in"
5. Tap "Accept" and the data should now be synced

NOTE: If you wish to set up a Google Apps account, tap "Settings" > "Email + accounts" > "Add an account". Tap "Advanced setup", enter your username and password, tap "Next" and make sure to tap "Exchange ActiveSync"

Sync the Google Account on the iPhone 6 / 6 Plus:

1. From the home screen, tap "Settings"
2. Swipe to and tap "Mail, Contacts, Calendars"
3. Tap "Add account" under "Accounts"
4. Tap "Other" and under "Contacts", tap "Add CardDAV Account"
5. In the "Server" field, enter "google.com"
6. Type in your Google account's log in credentials and enter the account name in the "Description" field
7. Tap "Next"
8. To sync your calendars, tap "Add account" > "Other"
9. Under "Calendars", tap "Add CalDAV Account"
10. Type in "google.com" in the "Server" field
11. Enter your Gmail address' password and user name and tap "Next"
12. Select to sync "Reminders" and / or "Calendars"
13. Tap "Save"