Which formate is best for Banners, supported by all!

1

I am creating banners in swf, but my client says that all the browers does not support swf files and pop up says to install flash player and this way he may loose lots of visitors. But if I convert it to gif, quality gets very low and not crisp and smooth. please tell me the best format for animated banners on web and how to maintain the good quality always.