There's no easy answer to that question.
The natural fibres - cotton, linen and silk all crease easily as does man-made rayon.
Wool doesn't crease so badly but then you probably don't want wool for summer.
The synthetic fibres, mainly nylon and polyester are much more crease resistant largely because they don't absorb much moisture, but in a single layer is unlikely to be comfortable, especially if you don't like poly-cotton.
The reason there are a lot of blends, like poly-cotton, is to try and get the best of both but it never truly works. Poly cotton is never as absorbent as 100% cotton and it's never as easy to care for as 100% synthetic. It's a case of weighing up the pros and cons and deciding which is most important to you.