iPhone 5 in black. NOT Space grey. The original black one that was criticized because it scuffed easily.
I still have mine, it's in pristine condition because it was kept in a case all its life. It's one of the sharpest looking iPhones ever released, probably only tied with the Leica camera-inspired iPhone 4 design. (Even Steve Jobs admitted it in the keynote.)
I agree about the space gray iPhone X w/black leather case. Maybe a step further I'd say the space gray XS Max with black Apple leather case. Makes the camera look even sleeker since the body is larger.
Personally, the iPhone 7 Plus in the standard matte black color always looked pretty amazing, at the time it was the only 'slate' black iPhone we got since the iPhone 5 era
I kept mine from new as long as I realistically could because I was just simply 100% happy with everything it did.
But in the end I had the option of either running an old-but-fast iOS which couldn't open any contemporary apps, or cripple it with more recent iOS that ran like a snail but could at least open contemporary apps.
Neither option won over the other so I reluctantly updated it to a then-current iPhone 6S (which last year I changed for an identical-looking SE2).
The 6S and SE2 somehow just never seemed as premium and exclusive as the 4S.
To this day, the likeness of the 4/4S is still widely used as a generic 'phone' image.