I've only been to Turkey and Morocco (multiple times to both)
I think in terms of cultural and scenic variety, Turkey comes out on top. Admittedly, the cultural variety can be hard to get a grasp on as enforced Turkish nationalism is quite strong; regional cultures and languages aren't always exactly encouraged, but it is there. There is a world of difference in terms of lifestyle between Istanbul and the East, and between different cities and regions (not always just an East/West divide). The landscape and climate is hugely varied as well. This is also true to an extent with Morocco, but it's a little less apparent. Then you have some truly stunning archaeological sites representing a number of different civilisations, some of the most beautiful coastline in Europe (south of Fethiye along the Lycian coast), great food (also true of Morocco) and some one-of-a-kind attractions.
I'd love to go to Egypt, and I'm sure Cairo would compare with Istanbul, but once you get away from the archaeological sites and beaches, is there much else to it to interest a casual tourist? The same with Jordan- you have Petra, but I've not heard much good said about either Amman or Jerash...and I doubt such a small country could contain similar variety to Turkey.
One of my biggest regrets is not getting to Syria before the war. I had vague plans to go, but it didn't happen.