Haas still has one of my favorite one-handers. Probably still my favorite. Mainly because of how Haas can hit that backhand under different sitautions. He hits it clean, he can hit it flat or with lots of topspin. He has a nasty slice backhand. And the true test--he can still hit an amazing one-hander off the backfoot if he has to, on the run, and even when he's off balance.
Richard Gasquet's one-hander is fun to watch, crazy angles. I haven't seen enough of Gasquet to make a call on his one-hander, but his running one-hander always impresses me.
Henin-Hardenne's backhand, not the most beautiful to me, but a really good model of how to hit a high backhand. Considering how short she is, but how much firepower and angle she gets from hitting a ball at head height... A common bane to the existence of a one-handed backhand drive (as opposed to slice). A very sweet one-hander in how she can drive a ball so (relatively) high.
I wish I had seen a decent amount of Sampras from the early and mid 90s. Sampras fans assure me that his one-handed backhand at the time was great. I only started watching tennis seriously in 2002, and while I admit his backhand then (when his game went more S&V oriented) was used really well as a tool... I wasn't impressed with it solely as a weapon. But I do understand he changed his backhand when he went towards S&V play. Would love to see more clips or even matches when he had his attacking backhand. Might change the way I look at Sampras, even.
-Chanchai