[js1138]

Bacteria is the most numerous form of life in terms of any number of measures: raw numbers of individuals, numbers of species, genetic range between different species, ubiquity in differing environments, and even, perhaps, biomass (the argument here relies on the empirically somewhat shaky speculation that bacteria prosper throughout at least the outer crust of the earth, fed by geothermal rather than solar energy—an argument needed to get their biomass over that of plant life, which otherwise easily dominates due to the weight of the worlds forests).

In terms of numbers, bacteria are by far the most numerous of organisms on Earth. They also make up most of the living biomass on Earth. Unfortunately, I'm unaware of any actual estimate of the number of bacterial cells on Earth. In fact, we don't even have a very good estimate of the number of different species of bacteria. What we do know is that there are over 1000 species of bacteria living inside of us, with 300 species of bacteria in a person's mouth alone, and that we have 10 times as many bacterial cells as blood cells in our bodies.

[From many - one.]

I don't think they form a class, though. See:

Prokaryotic Phylogeny - Taxonomic Ranking
http://www.bacterialphylogeny.com/taxonomic_ranks.htm

Or were you using "class" more colloquially?

[Doctor Stochastic]

In terms of biomass, bacteria, beetles, and earthworms are candidates for the most.

In terms of diversity of individuals and habitat, it would have to be bacteria.