General:
* Level: (especially for surfaces)
* Even: (implies uniformity)
* Plane: (a mathematical term, but can describe a flat surface)
* Unbroken: (emphasizes the absence of irregularities)
More specific:
* Slick: (implies a slippery smoothness)
* Polished: (implies a high level of smoothness, often due to work)
* Sleek: (suggests a smooth, elegant shape)
* Glossy: (indicates a reflective smoothness)
* Unblemished: (highlights the lack of imperfections)
Figurative:
* Effortless: (can be used to describe a process or experience that is smooth and easy)
* Seamless: (implies that there are no transitions or interruptions)
The best word will depend on the specific situation and what you want to emphasize.