Updated Jun 8
If they are alive on Earth today, then none of them are primitive.
Every species currently existing is a product of billions of years of evolution.
A small, relatively simple, single-cell microbe species will be highly adapted to its particular environment. They can thrive just fine, live out their lives, create progeny, etc.
They replicate very rapidly, and, as a species or a strain, can adapt fairly rapidly to changes.
A species can even evolve by simplifying itself. If you live in an animal’s gut, you might lose the genes to make certain molecules which are plentiful in that environment. You would actually become more efficient, and thus, more evolved.
Unicellular organisms may seem “primitive” from a very basic view, but they start looking very complex when you look closely enough. This applies to individual cells, and to populations of individuals interacting with each other.
It’s very much about context.
There are plenty of single-celled bacteria and archaea which thrive in environmental conditions which would kill a human very quickly and painfully.
From humans’ perspectives, microbes may seem primitive and inferior. From some microbes’ perspectives, humans are clueless, enslaved providers of food, shelter, and transportation.
Perhaps one of the most “primitive” organisms on Earth are those zombies who devote their biological structures, processes, energy, and time, to alternately watching reality television, and then compulsively staring at their phone to scroll through FaceBook.