I find that vertical always, always converts better. I also try to pay attention to which pins I like. So when you're pinning things yourself, which pins stand out to you and what do you like about them?
It is probably impossible to get you a perfect answer here as it always depends on product, branch, the picture itself and goal which is why I would alway always always suggest A/B testing. If you try different sizes and tag the links in the pin correctly, then you can see in your Google Analytics later on which formats, colours, themes etc perform best. I hope that helps.
I've found that vertical 2:3 is most often used and recommended, no longer than a 1:2 ratio or they will be considered too long and be cut off at the bottom.
Don’t create square images for Pinterest, I tend to create images at 720 (my blog’s content width) x 1080, or occasionally 600 x 900 for even tighter file sizes.
I had even started making my “regular” Pins taller than 2:3, because others have raved about how well tall Pins perform.
Hope it will help. 🙂