What species is most prolific?
This article covers your desires. Briefly, the single species that weights the most is the cow (520 million tonnes in 2011, now probably way more). About the most members, it looks like that the prize could go to the Antarctic krill (7.8 x 10^14 according to Wikipedia. [

This BBC article, instead, argues that collembola might be the most numerous group on Earth, being present in the number of 10000 per square meter (I leave the calculation to the reader). Collembola, however, are made up of ~6000 species. The same goes for ants and termites. 

Regarding bacteria, the same article mentions 
cyanobacteria, and more interestingly Wolbachia, an intracytoplasmic parasite that can bend the sex of the progeny of its host to spread more effectively.