Polyester fibre for fullness and support or feathers/duck down for that designer look - you may need to fluff/ puff them regularly and for comfort you sink into them. Great for the karate chop look

Spotlight have great feather inserts or pillow talk.

I bought some from Amazon last year and they still look fluffy.

I replaced all of my cushions for feather inserts for the designer look, and so I could ‘chop’ them. I have to admit I prefer polyester for comfort when laying on them.

Agree with others here, the down filling inserts from ikea have a great loft and sit beautifully. I put two in each cushion cover so they have lots of fullness which flattens pretty quickly to a plump but not over stuffed cushion

Feather inserts can be bought from places like Queen B - like suggested above make sure you buy them larger than your cushion cover that way the cushions are fuller. Feathers last for a very long time with a bit of care.