Which is the most common variety of coffee bean?

The most common variety of bean is Arabica, however Robusta coffee beans are more abundant than Arabica. Robusta beans contain more caffeine than Arabica does. This accounts for the difference in taste and also accounts for why Robustas are often regarded as medium to dark-bodied coffees while medium roasted Arabicas lose their complexity. You can check all about those two here: https://www.eraofwe.com/coffee-lab/articles/arabica-vs-robusta-what-is-the