There was general expectation that for ‘m’ there would be the Modern Defence (1…g6) which I played almost exclusively in the 1980s. As I no longer encourage this kind of thing my choice instead is the Marshall Attack, a gambit line against the Ruy Lopez developed by Frank Marshall. The following game is the best known Marshall Attack despite the fact that Marshall lost. Capablanca’s defence, when confronted with a new and unexpected gambit line, was superb:
There is a myth that Marshall saved up his gambit for 10 years before trying it on Capablanca but he had experimented with a form of it (Frere – Marshall, New York 1917 had gone 9…e4 10.dxc6 exf3 11.d4 fxg2 12.Bf4 Bg4 13.Qd3 Nh5 14.Bxc7 Qxc7 15.Qe4 Nf4 16.Qxe7 Qxe7 17.Rxe7 Bf3 0-1) before the Capablanca game and it seems it had been played even earlier in 1893. Marshall subsequently came up with the improvement of 11…c6, and this became the main line. Here’s a nice contemporary game which illustrates Black’s attacking chances:
Is there a problem with the Marshall? Well one major issue is that it is difficult to get in as White has many ways to avoid it even if he plays down the main line of the Ruy Lopez (8.d3, 8.d4, 8.h3, 8.a3 and 8.a4). It also has the reputation of being drawish when White plays into it and knows his stuff, and that does not appeal to gambit players!