According to dictionary.Com, a subspecies is "a subdivision of a species, especially a geographical or ecological subdivision." Scientists often disagree on the existence or non-existence of subspecies. Often the only reason that a subspecies exists is due to geographic isolation. Several thousand years ago, there were many distinct subspecies of humans. Over the last few thousand years, globalization has caused much interbreeding and blurring the lines between subspecies. However, that doesn't mean that subspecies don't exist, or that new subspecies couldn't develop over time.
Nevertheless, because this is a question about categorizing groups of people, it's a subjective question. Personally, I find observable differences across populations of humans that allow me to identify subspecies. I can reliably guess what region of the world a person is from just by looking at them. Anyone who would deny the existence of subspecies of humans would likely do so for political reasons. From a scientific perspective, their existence, to me at least, is clear.
Look at the evidence. Different ethnic groups have different bone structures and body types; some even have unique adaptations to the environment in which they developed. Tibetans, for example, have adapted to better breathe the thin air in the Himalayan mountains. There's no need to bring anything racist into this. Its just a fact of biology.
Ethnic traits vary among races just as they vary between races. It's ridiculous to consider human beings to even have "subspecies" in the first place. Never mind that this "subspecies" argument was used to justify some of the world's most horrific atrocities (slavery and the Holocaust for example). There's nothing wrong with differences, but these differences don't equate to "subspecies."
According to Merriam-Webster, subspecies must be genetically distinguishable from each other. Some ethnicities are genetically distinguishable, while others are not: consider the Han Chinese and the Manchurians. While they might demonstrates differences in the way they speak or dress, there are few, if any, major genetic differences between these two groups.