Colnago still makes them. That being said, anything from the 1990's with vertical rear dropouts (so you can use modern lightweight wheels) is a great choice.
Why vertical? Classic steel lugged bikes had horizontal dropouts, and the hubs of yesteryear typically had serrated steel contact surfaces because you needed it so that the wheel wouldn't "slip" forward. The system needed to be tight, so everything was beefy. Put some modern wheels in a classic horizontal dropout, and see just how quickly you can pull the wheel forward when you give it the gas.
Once bikes started getting vertical dropouts, the hubs could also change, and no longer required the brute force to stay in place. The hubs then started using lightweight components/designs. The vertical dropout is where I draw the line between classic and modern when it comes to lugged steel bikes. Not much else has changed.