Mike is right that there are too many blessings to pick one. In a word, salvation has to be it, to know God again, walk in his presence, enjoy his fellowship. So basic, that one.
But let me go to another blessing of the gospel, and that is the amazing privilege of sharing in the gospel’s proclamation. That we, as fallible mortals, have been made partners with God (see 2Cor 6:1 in NLT) in the task of communicating the salvation that has been offered to all mankind leaves me breathless at the goodness and wisdom of the Lord.
To know that the Lord uses me to bring another soul to salvation, that he has chosen every single child of his to enter into the labors and sufferings of Christ, that the joy of redemption is increased by our participation in God’s mission — this too is a blessing and a marvel and a reason for praise and thanksgiving.