What are the greatest blessings God has bestowed upon us, and is it correct to say that the greatest of these is that God is our Lord?

The greatest of Allah's blessings upon His servants is guiding them to His religion, by perfecting His religion for them and making them in no need of any other religion or another prophet. This guidance is the absolute blessing, for the sending of messengers and the revelation of books clarify the straight path, and whoever accepts it will be among the best of creation. The blessings of this world are not true blessings except for those who are grateful for them and use them to obey their Lord; otherwise, they will turn into a tribulation for them.