Now I’ve beaten all three versions of Street Fighter 3 with Yun. The easiest is probably 2nd Impact, as Gill is not even the last boss (Yun’s brother, Yang is), and he doesn’t have the nasty resurrect.
You can also see how the game becomes more “barebone”, as new characters are added. First background quality degrades with 2nd Impact, then with 3rd Strike there are no win screens anymore.