Counterpoint. F-15 is one of the (if not THE) most successful fighter aircraft ever made and it doesn't look so sleek in my opinion, relatively speaking.
I'm an aerospace engineer and I find the F-15 (esp. the C model) to be a hell of a sexy beast (so to speak), very visually stunning, and the F-35 to be relatively ugly. There's a reason the U.S. fighter community refers to the F-35 as "Fat Amy".
F-15 isn't sleek, but it looks mean and hard. F-16 looks sleek.