A seemingly simple question, that is amazingly difficult to answer.

Perhaps part of the problem is the amazing diversity of types of bacteria found in nature due to the high mutagenicity of bacterial DNA. And the belief that up to half of the living biomatter on earth are the millions of species of bacteria.

Only a small portion of bacteria are pathogenic to humans with many types leading a symbiotic and potentially beneficial life in, on, or around humans. Perhaps this makes the question even more difficult because skin flora differs from mouth flora which differs from gut flora, and all of it differs from environmental flora.

One note indicated potentially Escherichia Coli as being the most common, but did not give any data to back it up. Only a tiny fraction of the E-Coli fall under the classification of Enterohemolytic E-Coli (O157:H7) which is so often reported with respect to dangerous outbreaks. However, while non pathogenic E-Coli is very prevalent in humans and animal intestinal flora, it only makes up less than 1% of the total gut flora.
http://library.thinkquest.org/06aug/01942/bacteria/common.htm

Better data seems to be from the website:
http://www.earthlife.net/prokaryotes/welcome.html

Unfortunately, it is confusing whether they are discussing common bacteria by species and subspecies, or common by total population.

The Phylum Firmicutes is the largest grouping of bacteria with over 2475 species. The Firmicutes are gram-positive bacteria.
The Phylum Proteobacteria is the second largest grouping with over 1534 species. The Proteobacteria are gram-negative bacteria.

However, I would note that that one article suggests that 75% of known eubacteria are gram negative, which would NOT include the Firmicutes.
http://fig.cox.miami.edu/Faculty/Dana/monera.html

According to the earthlife pages (above), 27% of all species fall under 10 genera: Bacteroides, Corynebacterium, Streptococcus, Mycobacterium, Lactobacillus, Mycoplasma, Bacillus, Pseudomonas, Clostridium, and Streptomyces. Again, this is probably due to a distribution by number of subspecies.

For your gut flora, one article suggests that over 95% are Bacteroides, Bifidobacterium, anaerobic streptococci and Clostridium.
http://www.cehs.siu.edu/fix/medmicro/normal.htm

Another article suggests that Over 99% of the colon's bacteria are obligate anaerobes namely: Baceroides, Fusobacterium, Clostridium, Peptostreptococcus, and Eubacteria.
Normal Body Flora.doc

We seem to be missing the Enterococci which are also very common gut flora, and as mentioned above, the Escherichia Coli (E-Coli) only account for less than 1% of the gut flora.

Sorry this isn't too straight forward, but the bacteria in the world are a very heterogeneous group that are found throughout air, soil, water, and in living animals as well as on living plants. Many are somewhat specialized for moving from one host to another. And because of the ubiquity of bacteria in our environment, humans and animals have developed extraordinary complex immune systems to protect ourselves.

Don't be surprised if someone tells you they found Staphylococcus Epidermidis on your computer keyboard. It is all over your skin too!!!

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