As others have said, Dura-Ace 7700 GS, which I have on my Trek 720 right now. Works beautifully either friction or indexed. If I had to go with a different one, it'd be a Dura-Ace 7800 short cage. For friction era specifically, pretty much any upper-tier slant parallelogram Suntour gets my vote, even (if not especially) the Mountech/Superbe Tech stuff. Very smooth.