While "complex" is a subjective term, the most complex single-celled organisms are often considered to be the protists, specifically those belonging to the Stramenopiles group.

Here's why:

* Internal Complexity: Stramenopiles like diatoms and brown algae have intricate internal structures, including chloroplasts for photosynthesis, complex cell walls, and specialized organelles for various functions.

* Multicellularity: Some protists, like slime molds, exhibit a unique characteristic of aggregating into multicellular structures during certain stages of their life cycle, showcasing a level of complexity approaching multicellular organisms.

* Genomic Complexity: Protists often have larger and more complex genomes compared to bacteria and archaea. They have a greater diversity of genes and regulatory mechanisms, contributing to their complex cellular functions.

It's important to note that "complexity" in single-celled organisms is multifaceted and not solely based on size or number of structures. Protists, with their diverse evolutionary adaptations, represent a fascinating window into the incredible variety and sophistication of life at the single-cell level.