Always post to Instagram using Max Crop
When Instagram first allowed users to post in crop formats other than the 1:1 square ratio, everyone went crazy. You finally had somewhere to post all your full landscape crop photos in all their glory. It wasn't long after this that users figured out that this was not the smartest thing to do.
In fact, I'd go as far as to say that you should completely avoid posting in 2:3 crop. (This applies to both photo & video content). Here's why:
full landscape crop (2:3) sucks - on INSTAGRAM ANYWAY
User experience & screen real estate. Look at the photo sample above. It looks horrible on your phone screen & Instagram. It's small and takes up so little screen real estate that it looks like a preview thumbnail rather than a full photo.
Two Seconds. You probably have 2 seconds (if even) to capture your followers attention and convince them to like & comment on your photo. When people are scrolling through their home feeds, they'll fly past your small thumbnail of a photo and give it no attention (or likes).
Your photo will be competing with other users who are posting in full 4:5 crop. Their photos take up almost the entire phone screen!
Feature pages will avoid featuring your photo. I run @awesupply and trust me when I say this, it is a huge hassle to feature photos that are any smaller than 1:1 (square sized). Instagram doesn't make it easy. Rather than just screenshotting your image and posting it directly from the Instagram app, I have to screenshot it, exit the Instagram app, go to my photos app, crop it, then go back to Instagram to post it.
Is your photo worth the effort? Why not just post someone else's photo which is equally as good? Beyond that, landscape photos never perform as well (due to it's minimal real estate - see point above).
General rule to follow
I know it's difficult always posting in full 4:5 crop. Not all photos are meant to be displayed vertically, and that's fine. It just means that those 2:3 crop don't belong on Instagram. Post them to your other social media / photo sharing platforms instead.
To the very least, post to a minimum of the original 1:1 square format, never smaller.
If you don't trust me, go through you Instagram list and browse all the popular and "successful" Instagrammers on that list. You'll notice that most, if not all, are using the 4:5 crop almost exclusively!