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I believe b5 is worse on basis that after Bxb5 c6 Be2, white is up a pawn and black has no compensation whatsoever, whereas with f5, after exf5 Nf6, black has lead in development, center majority, and black also is likely to regain the pawn after d5. Nevertheless, votes shall be updatationed.
3... g6 or 3... h6 both respond well to that.
Maybe I'm feeding a troll again here but obviously those moves don't even "respond well" for White in the same position up a tempo after 1. e4 e5 2. f4 exf4 3. Nf3 g5. Also you have 3. d4 and 3. Be2 to deal with. If Black could regain the pawn it'd be a fantastic opening.
1... Kxd8 is actually the best move. No joke. Here's why:
1. e4 Kxd8!!! 2. d4 and from here, Black's king absorbs the queens power and now white will have a difficult time checkmating him.
Let's look at the other moves:
1... Nh6: bad, but not as bad as 1... Na6
1... h6: nothing's really too dangerous about this move.
1... a5: bad, but you can do worse.
1... a6: it is bad, but in a way, it is good, preventing Bb5
1... f6: weakens the e8-h5 diagonal, prevents the knight from going to f6, and does not help at all with development. However, no material is lost, the king's knight might go to d7 instead, and the pawn structure is fine.
Let's now go to the worse ones:
1... b5: losing a pawn, but, allowing a Bb7 fianchetto. This is better than 1... f5?!
1... Na6: bad, but not dangerous. And if 2. Bxa6!? bxa6 but now, black has the bishop pair and can do a Bb7 fianchetto.
1... h5: really bad. trying to develop the rook, but 2. d4 prevents it. Black should now consider castling queenside. I still think you can do worse.
1... g5: preparing for a fianchetto, and weaking the kingside. 2.d4 attacks the pawn. Black can defend it, though.
1... f5: 2. exf5 is good:
Because of this, I vote 1... f5 as the worst.
I think f5 has the small merit of black getting two centre pawns and white one. But for the rest f5 looks trashy. My vote is b5, losing a pawn on the spot and does not help development of minor pieces or controlling the centre.
Well b5 does help devolop the Bishop Bb7 and also might build the center with tempo against the White Bishop c6, d5, etc... They're both pretty awful moves. As I said earlier the central majority in the f5 line isn't a thing. People already admitted Black has to play d5 at which point the e-pawn is nearly backward and follow with e6 or e5 at which point the gambit is permanent, the central majority is gone, and black is simply down a pawn with a bad King position.
1... b5 actually helps with development, preparing 2... Bb7 attacking White's undefended pawn, allowing him/her to gain time.
Nobody's appreciated how bad Na6 is yet.
After 2. Bxa6, Black immediately has a double pawn on the edge of the board, and might as well be a pawn down.
Not as bad as f5, but I think it ties with b5.