GT4 was awesome in that you could go directly into the country from the main menu. Only 1 click and you were there. With GT Sport, you had to click on Brand Central, then the area (America, Europe, Asia), then the manufacturer. WAY too many clicks just to get to the cars. GT7 replicated this, and it's still annoying. HOWEVER, it's much quicker. That said, GT Sport was very clean looking with smaller menus that are off to the side or at the top. But, again, it made for more clicks and more time to get anywhere.
I did like the look of the menu in GT5 when they went to the boxes instead of icons; but GT4 was still cooler.
For the top spot, I'd have to give it to GT4. After that, GT7, simply due to the speed of navigation.