The player sprites is the one aspect of the game that all three console ports are the most similar. Even the Mega Drive version has the same amount of shading as the other two ports in almost every case and it looks like the sacrifice in background color was made to keep the sprites at full quality.
The PC Engine version has the most vibrant colorful sprites which look as good as or better than the Super Famicom version in all but a few cases. The Mega Drive version also has nicer color than the Super Famicom version in most cases as it's coloring was based mostly on the PC Engine version. Even the sprites (like Chun Li) which are styled after the Super Famicom version hold up very well. The Super Famicom sprites like the backgrounds often use drabber, sometimes more realistic tones which often end up looking like darker or color drained versions of the other ports.
The 3 weak sprites that stand out on Mega Drive are regular Ryu and both colors of Ken. For some reason, Sagat's alternate color shorts are missing shading on PC Engine and Mega Drive. The alternate color Dhalsim has dark clashing colors on PC Engine and Mega Drive while the Super Famicom version is colored like the Arcade. Ryu's bandana is half as thick as his head on the Super Famicom version.
You can compare all three ports as well as the Arcade in the following video:
You can compare the actual images of all of the regular and alternate color player sprites on the following pages: