Flash looks better and it is not always as heavy as you guys say)) Unless your site is going to be optimized for mobile devices of course. But it's not possible to please all the visitors with their variety of browsers and OS. So, I vote for Flash.
I prefer gif. but as you ask png or flash I am with png banners because flash banners are not friendly with all visitors. they increase page loading time also need pluins to run. I choose those methods which are friendly with all.
A simple PNG banner is fine, I'm sure. Who pays attention to Flash banners these days? I know most young'uns are probably annoyed by them, or have learned to ignore them.
(I do have to chime in that I always get the highest clicks on my site through simple text ads).
My vote goes to animated gif. Flash is ready to "die".
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PNG's for sure. These days with Ajax based sliders and what not that are cross-browser compatible. PNG's are smaller in size and higher in quality so you receive better performance and is cross browser compatible and cross device compatible (such as iOS)
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Who are these idiots who're saying 'mac', 'macintosh', and 'apple' don't support Flash? iOS doesn't support Flash, OS X supports it just fine.
In saying that, according to the trends in web usage more and more people are browsing the web using phone and tablet devices, the extreme majority of which don't support Flash (both iOS and Android), so despite the number of desktop computers supporting it or even running it Flash is a dead duck and unless you know with absolute certainty that an extreme majority of the people viewing your ad are going to be using Windows or OS X, it's not worth even considering.
At that point it comes down to whether you want to have static imagery (PNG) or an animation (GIF).