My Favorite Gambit: Vienna Gambit
This is the Vienna Gambit, A very tricky opening with the moves, 1.e4 e5 2.Nc3 Nf6 3.f4 and if they take your pawn on f4 you can play the move pawn to e5 forcing their knight to retreat.
In this Gambit not only have you slowed down their development,but you also have very good center control.