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