The question is so open ended as it depends entirely on the pallet you are working with. To come to the client with a set of standard “These are the best landscape design ideas for the backyard” is a recipe for a very ordinary looking garden
The design should be marriage with the architecture so it adds value both monetarily and to the lives of those who use the space
If it is a small space then using tricks to make it appear larger, darken the fences, tier the foliage, take the eye to the corners, create places you are forced to explore visually etc., while not compromising the need for a table and chair
If the space is too large, how do you make it cosy or direct your eye away from anything unpleasant surrounding the garden space, ugly houses or trees etc.
Each garden should also be a unique expression of the owner. This can be from the minimalist to the incredibly complicated (That is designed to appear simple J ) Do they want fruit trees of Cactus? Do they like soulless French palace gardens with miles of manicured hedges that show how rich they are (Or wold like to be) or a garden stuffed full of bulbs and flowering plants or even something in between
So the best design ideas, are the ones that work