I'm not 100% sure I'm allowed to say this, but I'm fairly sure the manufacturer that consistently has the lowest vulnerability to rowhammer, at least in DDR3 and DDR4, is Samsung. I think it'll be interesting to see how DDR5's "refresh management" impacts the attack, since it is implemented in the DRAM modules themselves, but the density (and thus sensitivity) is also very high in those chips, and they have very, very small access transistors.

Either way, the best solution is still to use ECC and set the computer to panic on the first uncorrectable error.

(Ideally, also panic on the first correctable error, although that would require a small kernel patch)