Can someone tell me what is the best ai Image generator for creating high quality png ?

For Photos and Artwork, Midjourney and Adobe Firefly are pretty good. 

It's not one-size-fits-all. 

What type of content do you wish to generate? 

Why must it be PNG?  

The majority are very good. And are equally bad. A lot depends on the type of content you wish to create. Some are best for cartoon and illustration. Others do a better job at creating photo realistic images and people. Most let you experiment with some free credits before requiring a purchase (which is necessary if you wish to sell your assets commercially). Experiment and see what works best for you.