Kicking this one back open, and holding to the SHS only rule I'd say post 3050, the Gauss Rifle would probably be THE standard weapon for 'Mechs. In a world without DHS, sure it's heavy, but you can really do a lot with it. The LB10, just as much, but I can't see the kinds of energy boats we get later in most cases.
Maybe something like an HBK-4P with an XLFE, the same heat battery, and a barrage of MPLs instead of regular MLs.