It's called the Halloween gambit.
The best gambit

Well, if you want to split hairs the Queen's Gambit as such given that the c pawn can be recovered by force.
As to what is the best gambit, I don't think that that is a question that can be answered objectively with any certainty. Anytime you truly come to gambit a pawn or a piece there is some risk involved and it is not just a risk based upon known theoretical principles either. If you do not have enough ability to generate an attack and sustain that attack for an advantage going into the middle and or endgame then gambitting anything is a recipe for disaster
I found a really chess opening that puzzle a lot of people. Excellent gambit