I think the 43 and 45 are special in their own ways.
The 43 is a perfected 42 that retains all of its design characteristics, including the prehensile function that makes the suit-up scenes vastly entertaining to watch. It reverts back to the classic color scheme of predominant red with gold, which I appreciated. Its design looks really mechanical, which is appropriate considering it flies around in separate pieces and assembles itself around Tony. We got to see it in action the most during Age of Ultron, and it is presumably the only armor that is compatible with Hulkbuster. Also, we finally got to see the shoulder-mounted anti-personnel weapons again, which was probably my favorite Mark 43 scene in the movie.
The 45 is an entirely different design. The hexagon around the arc reactor was a subtle but cool aesthetic element. The armor's design is sleek and incredibly organic-looking, closely resembling the human musculature and giving off an "android" appearance. Due to its organic look, Tony and the armor seemed even more intertwined...he literally looked like a man made of metal. I personally thought the armor fit nicely into the movie's theme of evolution and loosely paralleled Ultron's quest to make better and better bodies. It got a large amount of screen time at the end of the movie, but still not enough in my opinion. We were unjustly denied the suit-up scene, which I'm sure would have been really cool to see...maybe it has the prehensile function as well, but we'll probably never know. I also appreciated the return of the lasers, which Tony used multiple times throughout the final fight scene.
Both armors are special. In fact, they're my two favorites of them all. I have a hard time deciding which one I like more. To each his own, but I think the Mark 45 (and the Mark 43) both look really freaking cool.