There are so many cool planes, it is hard to choose. Maybe I will categorize it a bit.
My favorite Russian aircraft is the MiG-29, hands down. Everything about it is cool.
It is a much harder task to say what my favorite American plane would be. I would probably go with the A-10. I looks awesome and packs a big punch. Plus, it has one cool gun in the front.
My favorite British plane would be Harrier. It is so versatile. It is pretty cool to watch it take off or land vertically.
