Anything half decent and not CEN.
You do all know that the flex characteristics of steel are the same, fancy stuff only gets stronger (not stiffer) enabling it to be made lighter?
Back in the day Columbus, Tange and True Temper were big names, not sure why Reynolds seems to have it sewn up, in the UK at least.
I spec’d my Shand with Columbus Life, it’s 1.8kg iirc, rides light.
Lightest steel frame I’ve ridden was a 1.6kg road Dedacciai, Salsa claimed 1.35kg for one of theirs which I don’t believe.