Which is the best OS X "TO DO"-app that I can sync my Android device with?
I recommend Wunderlist. It has apps for Mac OSX, Windows, iPhone, iPad and yes Android. It can sync all between these different platforms. By the way it's free.
Try out Evernote. It is free. I am pretty sure it can sync between Android and Mac. I cannot test it myself - I do not have an Android device.
Try iCloud Calendar and Reminder sync I developed it.Trusted with more than 200k downloads.