I think Federer hits way more topspin on the backhand than people think. Look at this;
...almost a reverse-follow through backhand.
This is probably the one true advantage of a one-handed backhand - ability to generate more topspin because of the longer and free-er stroke. 2-handers almost all hit it flat.
Anyway, in terms of the other pro's Wawrinka always looks to me like he hits it flat, and Kohlschreiber, despite having an extreme grip, rarely follows through with the forearm supinating, therefore he hits quite flat too, yet is stable on high balls.
Almagro and Gasquet's look loopier in general. Federer surprisingly seems to hit as much topspin as them, despite having a much more conservative grip.
Not sure about Kuerten. Does anyone have links to HQ footage of him? He is very interesting as he's said to be the only player to use a full semi-western backhand grip (most extreme backhand grip). Most players with extreme 1hbh grips use an in-between of an eastern and semi-western grip - therefore Kuerten's backhand may well have had the most spin of them all.