Curtains that are thick, touch the floor and have no gaps around the edges, are most energy-efficient.
They’ll also be even more effective, if they’re paired with a pelmet.
Without a pelmet, hot air rises to the top of the curtain, touches the cold glass of the window, causing the hot air to cool down. This cooled air then enters the room, making the room feel colder.
Pelmets prevent hot air from touching the cold glass in the first place, making your curtains much more effective.
If you don’t like the look of pelmets, you can purchase ‘invisible pelmets’, where the pelmet is placed on top of the curtain rod and is not visible from the front.