For hair:
* Straight is the most common opposite.
* Wavy can also be considered an opposite, as it's somewhere between straight and curly.
For other objects:
* Straight is still a good opposite for things like lines, shapes, or paths.
* Smooth or flat might be appropriate opposites for something like a surface.
* Simple or linear could be opposites for something complex or non-linear.
Ultimately, the best opposite for "curly" depends on the specific situation.