What modern organisms are thought to be most like the first life-forms on Earth?
Probably either cyanobacteria or archaea, both of which flourish today in all kinds of wet environments.
There is a presumption in the question that the earliest life-forms on Earth were what we would call organisms today. Depending on your definition of "life-form", pre-cellular arrangements of molecules may qualify as life. Different authorities use different definitions.
The earliest unicellular forms of life of which I am aware are still living today, namely cyanobacteria and archaea. The classification of archaea into phyla seems to be in a state of flux - or at least contention.
It is difficult to distinguish whether the earliest evidence of life is from cyanobacteria or from archaea, but evidence has been found dating from between
Prior to cellular life-forms may have been a primitive form of life based on self reproducing RNA strands. This is essentially the "RNA world" hypothesis. Prior to that, there may have been even more primitive self reproducing molecules.