What is the best paint/stain for treated lumber on a deck

Something with a preservative in - Shed or Fence stain.......

First, I'd strip off all the old paint, sand the wood, then put on 2 coats of a Maple colored Varnish {or any color of your choice}, let it dry between coats of course, then after 2nd coat is completely dry, completely cover it with a clear Spar Varnish. Mine has stayed in perfect condition for 4 years; I'll use the Spar Varnish on it again this summer after the weather is warm and there's no rain expected and every 5 yrs thereafter. Spar Varnish is used on the outside of boats so you know it'll last in just rain, snow, etc. Hope this helps.

Forgot to tell you to make sure the Varnish is for exterior use. That's all I use, even for the indoor furniture so I forgot to mention it.

Are you cleaning your deck surface with a pressure washer or any other cleaner before trying to paint? If it’s dirty at all the paint will keep peeling no matter what you use! But if it’s clean and still peeling, you’ll want to remove the paint you got on it now with maybe a pressure washer or whatever you want to use to remove anything that’s not stuck down well? Bher makes a really good deck paint that I personally used on some raised garden beds I made a few years ago and the paint still holding up very well in all the weather been throwing at it with rain, sun, hot or freezing cold and snow and it still looks like new! No peeling whatsoever! I didn’t put no primer on first either! Which may be another choice for you before painting is use a good primer to give the paint a better surface to stick to with your paint! Hope that helps? Good luck