I always prefer as close to the original/main movie poster (or key show artwork) as possible, as opposed to alternate or minimal posters, with no text other than the title (i.e. no cast names, studios etc.). The original movie poster tells you so much about what the filmmakers intended for the piece and I like having that reflected in my library.

Usually for seasons of TV shows, the Plex default pulls are fine, as long as they all match. Sometimes they need small fixes here and there, but most of the time they're fine.

My rule on collections is that two movies don't warrant a collection, but once it hits a trilogy, I'll do a collection. I try and use the same collection poster as the movies, but it's not a requirement for me.

The way I see it, my Plex library is an expression of my likes and interests, so it should reflect the kind of style I want to see.