Well, I would pick the JASDF F-2, but seeing that it is considered an F-16, I guess it wouldn't count. So, I'll say that the prettiest non-Viper aircraft would have to be the F-104. It was one of the sleekest fighters ever built, after all, it was once known as: "The missile with a man in it".
But as far as new generation aircraft are concered, I'll choose the Su-47. By far, it's the only Russian-made fighter I'm a fan of. Although I picked the Su-47 and the F-104, I do think the Gripen, Raptor, and the Typhoon are also sweet looking aircraft.
You've just gotta love that design.