I vote for the Edicaran fauna.
Most macroscopic fossils are morphologically distinct from later
life-forms: they resemble discs, tubes, mud-filled bags or quilted
mattresses. Due to the difficulty of deducing evolutionary
relationships among these organisms, some palaeontologists have
suggested that these represent completely extinct lineages that do not
resemble any living organism. One palaeontologist proposed a separate
kingdom level category Vendozoa (now renamed Vendobionta) in the
Linnaean hierarchy for the Ediacaran biota. If these enigmatic
organisms left no descendants, their strange forms might be seen as a
"failed experiment" in multicellular life, with later multicellular
life evolving independently from unrelated single-celled organisms.
No-one can say what the DNA of the Edicaran organisms were, or whether they were related to what came after. Apparently these fossils do not resemble the fossils of the subsequent Cambrian or much of anything else. The triskelion fossil I put in here looked the coolest to me. If they were a "failed experiment" or some sort of false start on multicellular life, they might have been totally alien.