What brand is the best quality exterior house paint?

I would suggest Sherwin Williams. Sherwin Williams duration is long lasting.

I have had very good results with Home Depot’s house brand (Behr). But a lot of it has to with climate and how good the painters are who put it on.

Most important thing for long life of paint is surface preparation ( sanding, priming and removal of loose paint).
Don’t buy “contractor grade” paint.

I think Sherwin Williams is the best for all paints.

What @Tropical_Willie said.

I bought a good grade of Behr paint a few years back to paint a porch. We specifically went looking for an exterior paint that said it was designed for to resist mildew and dampness. We also bought a good primer. Applied it all as called for and it covered well and went on nicely but I still have a mildew problem and Behr says they don’t understand why and cannot suggest any solutions. I was not looking for them to pay to have it redone or anything, just give me some suggestions.

Probably is, as @elbanditoroso suggests, an environmental problem.

@rojo I live in Tacoma where the pea soup-like weather greatly facilitates moss, algae, fungi and lichens to rule. As such they cause a green-sheen and black slime to form on just about everything in a short time. I don’t expect the paint or anything to stop that permanently. Each year I scrub it away with brushes, water, moss-be-gone, diluted bleach and elbow grease. Real fun stuff. HA! It’s a never ending losing battle. I’ll be very happy to find a good paint that can stand up to that abuse for at least 10 years.

Thank you all for you help!