Which is the best way to sync a Mac's address book with Google Apps Contacts?

Using an iPhone you can sync Mac<->iPhone<->Google Contacts.

The problem with duplicates is often syncing with two different services (iCloud + Google Sync + Soocial + Plaxo etc) or an import/export that goes wrong ie importing tons of duplicates. The hard-earned advice I have is to create one master address book by importing everything there, deduping it (if on google contacts use the "merge" function, or scrubbly), temporarily deleting the other address books (desktop/mobile) and then using a single sync platform and syncing everywhere.  My personal choice is google sync.  Watch out when you're setting up a new device, especially an iPhone as they will often ask you if you want to use a sync service ie iCloud and if you click yes, you could be creating another conflict/loop which can lead to duplicates or worse. I've laid out my hard earned-advice here and http://blog.writethat.name/how-to-clean-up-the-contacts-on-your-iphone-address-book/, and I must say it was the best 30 minutes I've ever spent productivity, tech-wise!

There are a few 3rd party apps that will let you sync Google and OS X contacts including groups. In the interest of full disclosure, I am the developer of the most popular one called “Contacts Sync for Google Gmail.” Here is a link to the app:https://itunes.apple.com/us/app/contacts-sync-for-google-gmail/id451691288?mt=12

Without question, having researched this extensively, the most foolproof method is Plaxo. I've been using it for 8 months to sync Google Apps for Bus, Outlook (7 and XP), iPhone and iPad on 9 computers/devices. Be prepared to pay about $60 per year for Syncing, plus more if you want a form of autoupdating peoples info called Personal Assistant

I'm now trying Spanning Sync http://spanningsync.com/ It seems to work better than Sync Man, but leaves many blank entries.. just deduped by contacts and seeing if it makes a difference.