This question has no answer because there is no real measure of complexity. You can define "complex" in any number of ways. For instance:
- Largest number of organs.
- Largest number of cell types.
- Longest string of DNA.
- Most genes.
- Most unique genes (not counting duplications).
- Largest number of different proteins.
And, undoubtedly, many many more.
Complexity isn't something that is obvious. For instance, plants are structurally simpler than animals, but chemically more complex (that is, they have far more chemicals in use in their systems than we do). Fungi are even less structurally complex than plants, but some of them have even more complicated chemistry!
"Complexity" isn't really something scientists talk about objectively.
However, Eukaryotes are generally considered more complex than prokaryotes and mammals to be more complex than non-mammals, such as reptiles. One could make the argument that humans, due to our enormous diversity of behaviour and effects on the world and other species, are the most complex. We are not as complex, as the previous statements mentioned, as many other organisms in the world. This question is very qualitative, any answer would just be opinion. A nice quantitative question very similar to this is: What organism has the longest DNA?