FWIW, It's sort of a mix of all of them.
The first thing to remember is that the timeline of the Star Wars Legends universe is immensely long. In terms of geopolitics, its earliest fixed point is the foundation of the Galactic Republic about 25,000 years before the Battle of Yavin, when the Humans and Duros were able to force unification on all the space-faring races of the Core sectors of the galaxy in the chaos that had followed the collapse of the Rakatan Infinite Empire. This collapse was caused when the non-human Rakatan's synthetic Force-using technology became increasingly corrupted by the Dark Side, leading to a racial mass-psychosis (imagine a species who were all behaving like a Sith towards each other).
The following 25 millennia is marked by a series of long, stable epochs under some form of republic punctuated by a collapse into galactic war. These wars were usually in some way characterised by conflicts between acolytes of order (the Light Side of the Force) and of chaos (the Dark Side of the Force) who, no matter what name they chose, were recognisably Jedi and Sith. The actual 'Sith' were the ruling dynasty of warrior-priests from the planet Korriban who were all-but eradicated at the end of the catacylsmic Great Hyperspace War of around 5000BBY. Many of the Dark Side factions seem to have been in some way empowered or influenced by remaining Dark Side-corrupted Rakatan artefacts such as the Star Forge, misidentified by many as the planet Exogol.
The Mandalorians first appear in galactic history in about 7000BBY under a warlord known only as Mandelore. It is not certain if they actually originate from the planet and sector that bears that name. However, they were also something of a cult-like multi-species culture, embracing as brothers and sisters any who would adopt their strict code of martial honour, chauvanism and conquest. From their first appearance in the galactic stage in the 8th Millennium, they were imperialists, seeking to unite the galaxy in a war they called The Great Mandelorian Crusade. They were rightly feared for their Viking/Klingon-like culture deifying war, conquest, pillage and slavery for all who were not of their culture. The Empire reached its high point in the 5th millennium when, during an attempt to conquer the Teta Sector, they encountered a rogue Jedi, now styling himself as a Sith Lord, named Exar Kun. Kun killed the then-reigning Mandelore and claimed control of the Mandelorian culture for himself. As succession-by-single combat was a long tradition, no-one questioned his right to do so.
This represented a cultural disaster for the Mandalorians, who were used by Kun as his disposable shock-troops throughout the resulting Great Sith War. After Kun was disembodied by the Jedi Master Nomi Sunrider, the Mandelorians were led a series of false self-declared 'Mandelores' in charge who led them in increasingly purposeless and destructive 'crusades'. It was whilst countering these increasingly-purposeless genocidal pillages (twenty years after Exar Kun's effective death, in the early 4th Millennium) that a Jedi Knight named Darth Revan was driven into the arms of the Dark Side by the horrors the Mandelorians were inflicting on innocents. Revan would come the closet any Sith Lord before Sheev Palpatine would come to ruling the galaxy, all but annihilating the Jedi Order and crushing the Republic (along with the Mandelorians, for whom they had a special hatred). Revan stripped the Mandelorians of their military might, forbade the wearing of their traditional armour and restricted them to their ancestral homeworlds in the Mandelore sector.
It is in this post-Revan form that we see the Mandelorians in the Clone Wars animated TV series. Although many cults like the Death Watch seek to restore their pre-Revan ways, most of the documentation is lost and the traditions forgotten and they are little more than pillaging thugs deluding themselves that they are some kind of romantic knights of a lost order of chivalry. The memory of their enslavement by Exar Kun and their humiliation and near-obliteration by Darth Revan have left deep cultural scars and those of the Mandelorians who seek to restore their people to the Old Ways despise Force Adepts of all traditions. However, the vast majority of the Mandelorians (descended from the human and human-like clans of the old Mandelorian Empire) are a modern galactic civilisation with a ruling council appointed by the clan leaderships, a strong civil society with law enforcement and universal education and live mostly at peace.
Sequel Trilogy AU
Daisy Ridley has recently confirmed that J J Abrams' original plan for the Sequel Trilogy called for Rey to be the granddaughter of Obi-Wan Kenobi. The only woman with whom he had a sufficiently close relationship for there to have been a child was Satine Kryze, the Duchess of the Mandelorians during the Clone Wars (they met several times during their respective youths and she was very much Obi-Wan's Padme Amidala). It is believed that their son, Rey's father in this original version of the character's story, was Korkie Kryze and that The Mandelorian may turn out to be that young man, hiding from his many enemies under a false identity.
If he is Korkie, then the Darksaber, the ultraviolet-bladed laser sword seen in the last episode of season 1, is his ancestral birthright. Yes, I know, the magical sword of the king. Since when has Star Wars been able to resist something like that?
