From what I could find, the best answer would be Ants !
What is the most dominant species on Earth? (hint: its not humans)
Nematodes are the most numerically abundant animals on Earth. Humans are the dominant species. We rule the Earth.
Dominant: More important, powerful, or successful than most or all others.
That sortof depends on how you define "species". If you accept that bacteria and archaea are "species" then probably one or other of those would probably win. (You can see the problem with species definition for bacteria and archaea with a quick look in wiki).
Also, as JB Haldane once said, "The Creator has an inordinate fondness for beetles", there being more beetles than pretty much all other types of insect put together.
But, in reality, Hippy's answer is probably most accurate.
Microorganisms such as bacteria and viruses and diseases. They evolve and adapt to anything that we try to fight them with at the drop of a hat. They spread easily and are hard to stop over long periods of time.
That possibly depends on whether you're confusing "dominant" with "most numerous".
While it's almost correct to say that humans are the dominant species it doesn't go quite far enough. Women are the dominant species. They rule!