2.. f5!? or maybe 1..c5 😀
asking for a "best" line is silly - there are double edged lines where black strives for winning chances, and there are quiet lines where black hopes white will get bored, resign spuriously and go to the pub. which would you prefer?
3..Nf6 (the berlin) is soliddy solid.
3..a6 is good, with 4..d6 and the idea of b5 at some point, grabbing space on the queenside with tempo. this isn't a line, more of a general motif
3..f5 is good fun, black can often go on a kingside sacrificial rampage, though its "theoretically dubious" whatever that means
The Classical Defence - keep it simple (Chess is a simple game).
it's off the beaten track, develops a piece, preps 0-0 Spassky has
some fine wins with it at his peak. it also carry a wee trap ot two.
What more do you want from a Black opening?
(I think Bronstein said that about this opening).
1. e4 e5 2. Nf3 Nc6 3. Bb5 Nf6 4. O-O Bc5
the line 1.e4 e5 2.Nf3 Nc6 3.Bb5 Nd4 is quite annoying sometimes, and also the moves for black are a little easier to make (one of the key advantages of the ruy for white is that black has all sorts of moves to try against you but it is quite difficult to pick the best move when there are several sensible alternatives for almost every position)