There is no accurate answer , as you say we talk about candidates . there is lots of strong theories about the origin of self replicators . RNA world was a strong candidate since latest experiments that suggest there is a good chance that some molecules that have simpler structure than RNA , could have a chance to do self replications .
there is also strong theories such as :
1.Prions that delivered by space objectives and Meteorites .
2.Life's origins may result from low-energy electron reactions in space:
first self-replicator recreated in lab:
RNA, or something very like it, has long been a strong candidate as the first self-replicating molecule in the origin of life. This is because it can both catalyse chemical reactions and carry genetic information.
But chemists first needed to explain how a large, complex molecule like RNA could form spontaneously to begin the process. They had done so for some, but not all, components of the RNA molecule.
The origin of replicators and reproducers:
you can also check the topic :The RNA World and the Origins of Life
from the book :
Molecular biology of the cell by Alberts B, Johnson A, Lewis J, et al.
after all this article was so innovative for me . I think it's a good explanation for self replication of molecules simpler than RNA.
"Maslov and Tkachenko's model imagines some kind of regular cycle in which conditions change in a predictable fashion—say, the transition between night and day. Imagine a world in which complex polymers break apart during the day, then repair themselves at night. The presence of a template strand means that the polymer reassembles itself precisely as it was the night before. That self-replication process means the polymer can transmit information about itself from one generation to the next. That ability to pass information along is a fundamental property of life"