It's impossible to definitively say which prokaryotes are "most common" as their abundance varies greatly depending on the environment you're looking at. However, some of the most common and abundant groups of prokaryotes include:

Bacteria:

* Proteobacteria: This is the largest and most diverse bacterial phylum, containing a wide range of organisms including E. coli, Salmonella, and nitrogen-fixing bacteria.

* Firmicutes: This phylum includes many important bacteria involved in human health, such as Lactobacillus and Clostridium. They are also found in soil and other environments.

* Actinobacteria: These are known for producing antibiotics and are found in soil, water, and even in extreme environments like hot springs.

Archaea:

* Euryarchaeota: This phylum includes extremophiles like halophiles (salt-loving), methanogens (methane producers), and thermophiles (heat-loving).

* Crenarchaeota: These are also often found in extreme environments, particularly hot springs and deep-sea vents.

Factors Affecting Abundance:

* Environment: The types of prokaryotes found in a particular environment are heavily influenced by factors like temperature, pH, nutrient availability, and the presence of other organisms.

* Ecosystem: Different ecosystems have their own characteristic prokaryotic communities. For instance, the human gut microbiome is dominated by certain bacterial species, while the ocean depths harbor a diverse array of archaeal populations.

Note: This is just a general overview, and the specific types of prokaryotes that are most common can vary significantly based on the specific environment or ecosystem you're interested in.