As lovely as bought furniture is, if you are tight on space it has got to be built-in, and it's worth splashing out and going bespoke. Here are a few examples of successful space user-uppers that will make any apartment, however cramped, become more liveable.
Stairs are always the most obvious waste of space, and turning the treads into draws or boxes and any cavities underneath into slide-out vertical draws can give you the equivalent of an extra wardrobe and chest of drawers.
The staircase above is fascinating, instead of having 2 sets, you use one on a hinge.
If you have the luxury of high ceilings, then going up is always the best use of space.
I'm bookish, so mini libraries like this would be really useful, but obviously you wouldn't have to fill them with tomes.
Whilst it's a massive pain to make a bed that is built in, especially when it's against 3 walls, it's an easy way to insert draws below and shelving above
And never be scared to insert shelving around windows, if done correctly it won't suck out the light, and you can always put mirrors around the edges to reflect more light into the room.
Several household items are better suited to vertical, pull-out shelving. Shoes, ties, and spices being the most obvious.
Doors you don't use very often can be made into recessed units (but this is expensive and means they'll have to be reinforced and they'll be a weight limit)
Built-in dining areas do save space, but I think the inflexibility outweighs the benefits.
And a few other ideas: