For me, I find all the Origins worthy of joining the wardens, but I think there are a lot of angles we can look at.
So, limiting it strictly to before they are recruited by Duncan, I will discuss each origin.
The Dalish: one of the groups who's religious beliefs are one of the most fascinating, especially after Inquisition, this is also one of my least favorite origins. It's short being a large reason for that.
But the dalish has his valleslin and passed into adulthood. I know that this was disparaged on the first page, "yay, he's an adult, so what?" but I think that actually overlooks a major part of it. The Valleslin is an extremely painful process. It's how you show you are an adult, but even in the process of receiving it, if you wince or react to the pain, it will be called off as you will be seen as not yet worthy of adulthood. Meaning that the Dalish already has a very high pain tolerance. The Dalish is also a highly skilled hunter, and is very reliable, if only to keep the members of the clan alive, either from natural threats or human ones. Then the Dalish gets tainted and, yes, survives due in no small part to Merethari, but has already shown a strong resistance to the taint, thus out of all the Origins, is the one that actually has the highest chance of surviving the Joining out of all the possible Wardens. Add in some experience fighting darkspawn in the forest, as well as tainted creatures, that adds points into the equation.
Survivability, reliability, hunting and combat skills matched with actual experience against the darkspawn makes the Dalish a worthy Grey Warden recruit. On a level of 1-10, with 1 being the overweight greedy town idiot who can't fight (Lloyd) and 10 being the most awesome recruit in every aspect, I would say that the Dalish is a 7.
City Elf: A looked down upon, second-class citizen. Living in an area where wielding weapons is illegal and secretly training with their mother, the city elf is probably the only origin that has absolutely no combat experience whatsoever until after the wedding. It's my third favorite origin.
Yet Duncan came to the alienage to look into recruiting us, and that is actually why the Hahren sped up proceedings for the wedding. From what I gathered, he didn't want to give up anyone to the Wardens so he sped up plans for the arranged marriage that were already in place to keep Duncan from getting a recruit.
That means Duncan knew about our training. But after Vaughn and his friend kidnap all the women, the city elf fights through the keep and kills Vaughn with some borrowed weapons, unless the player is a jerk and takes his bribe to leave. No combat experience makes this feat a lot more impressive. But from when I last played the origin, the impression I got was that the guards were taken off guard and they weren't the best trained or Denerim's finest at all. Vaughn's father, Bann Kendalls, had gone to Ostagar and had taken most of his men, and Vaughn is in the same situation as Cousland as, left in charge of the keep with only a handful of men. Still, taking down armed and armored soldiers and a few nobles who have training is no mean feat.
It's a great underdog story that really tugs at the heart strings. Add in Duncan coming to look at you as a possible recruit through knowing your mother and her skill, the ability to fight through a keep and some overconfident nobles just screams raw potential.
For fighting ability and raw potential, I'd give the city elf an 8. But the lack of education and lack of overall combat experience drops it down. Overall, the city elf is a 7.5.
Human Noble: Well trained, some combat experience in tournaments, the best education that money can buy and the potential for a lot of connections. Responsible enough to be trusted with running an entire teynir while the father and brother are away.
The human noble is a really good pick. Skilled in combat, already seen as a person of great worth simply by having a mabari loyal to him/her, dependable and raised to be responsible and dutiful. Fighting their way through an invading force while keeping his mother alive, possibly rescuing servants, the human noble is one of the best of the best Ferelden has to offer in sheer talent.
Because of their training, education and possible connections, the human noble as a recruit is a solid 9.
Mage: One of the best the Circle has to offer, the mage is the star pupil of First Enchanter Irving. May be highly skilled in any of the schools of magic if the player wants to specialize, or more rounded out if you want some spells from each two or three schools. Highly versatile in their ability to use magic, add in the rarity of mages in conjunction with everyone else, as well as the fact that there are only 7 mages currently fighting at Ostagar and none of them are wardens, the mage has a lot to offer. The mage, also by virtue of living in the tower, possibly has the best education among all the origins.
But the mage, like all other mages, is socially limited. Cut off from the outside world, the only experience he/she would have is helping Jowan escape by fighting a few sentinels and deepstalkers, and possibly some spiders. Deliberately denied going outside, even for exercise thanks to Anders escaping once, the mage will also be woefully out of shape.
One of my favorite classes to play, I think the mage is actually one of the weaker ones, in terms of what they can offer to the wardens initially but also has one of the highest levels of potential compared to the others, with the city elf probably being the only one that surpasses them. The mage is a 6.5, 7 tops, at the time of being recruited, but a potential 9 depending on the mages education, say combat tactics or the ability to make medicine through herbalism or something like that.
Dwarven Commoner: A casteless who legally does not exist in Orzammar. Forced to work as part of the Carta as an enforcer, this rough and tough warden recruit probably is the most practical of all the origins, and is less likely to see things in rose colored lenses than anyone else, like Alistair sees the Wardens.
No formal training, but already one of the toughest people in Orzammar due to them fighting in the proving against the best of the warrior caste and winning, then fighting through a hideout of other violent criminals and killing his former boss, the sky is the limit for the casteless.
No education whatsoever, it's probably a miracle he/she can even read, and if they can it would be because Rica taught them as she learned from Beraht on how to be a good noble hunter. Even though this origin never fights darkspawn, I actually think the dwarven commoner can have darkspawn fighting experience, especially if they did an odd job in the deep roads like Bodahn occasionally hired casteless to do, or catching nugs like that one casteless.
Sheer combat ability makes the casteless stand out. Add in a resistance to magic because of the fact that they'r dwarven and emissaries have a harder time just for that. For combat ability and experience, 9. But again, like the city elf, due to lack of education, connections or leadership skills, the casteless drops a bit. I consider a casteless a solid 8.
Dwarven Noble: As has been discussed a lot, this origin has it all. The education, combat training, combat experience and even leadership experience. The dwarven noble is being made a commander of Orzammar's army, and is even rumored to be a stronger candidate to the throne than the current heir-apparent Trian. Popular with the people, nobles and commoners alike, we know that the dwarven noble can be charismatic.
This origin also has the most experience with darkspawn. They simply excel at everything. 10.