Mage: Duncan told Irving he wanted a recruit, Irving pointed to one mage better than average, you, the one who just passed the harrowing easily. Mages are really good for the wardens so this is a no brainer.
Dalish Elf: Duncan comes across you accidentally, which is very weak. What was Duncan doing wandering the woods? He also recruits you simply because you survived the taint, not because he saw you fighting with great skill or anything. This whole story is basically that "fate" decided that duncan found you before you died. I guess that it makes some sense if you imagine that Duncan was searching for a dalish recruit on purpose (maybe he wanted a good archer), then accidentally came into you.
City Elf: Duncan wanted to recruit your mum but he could not, so now that he needed recruits he came for her son. I really doubt that city elves are that good at fighting (they are not even allowed to posses weapons!) so Duncan coming here feels a bit odd. It could be justified by thinking that after Duncan got Daveth, he went to the alienage to check out for you. He saw you were capable by defending the women, and recruits you. It also makes sense that no one will care if a warden conscripts a lowly elf, plus as you are fereldan, you have great motivation to save ferelden for this incoming blight.
Dwarf Noble: Duncan just happened to be there in the deep roads the moment your pc was exiled, this is another case of "fate" deciding your recruitment. Bhelen is shown as a master politician, yet he allows Gorim to see you and feed you vital information. The dwarf noble is also a pompous prince who has been treated all his life as royalty, he has just received his first command due to being of royal blood (not because of military exploits). It also makes no sense that duncan is hanging around in the dining room instead of looking for recruits, in all other origins he is doing it, so this feels disconnected, like here he is like: "nah i´ve got all the wardens i need, i am just chilling here and i will go the deep roads later to search for stuff".
Dwarf Commoner: Duncan went to Orzammar explicitly to find recruit, so the king calls a proving and duncan (being a former thief himself) decides to recruit the casteless who won it, because the casteless has just proven that he is a skilled fighter and very bold to commit such impersonation knowing the punishment. Then you fight your way out and kill the gang boss, showing Duncan that you are even more resourceful. Duncan says literally: "You are a survivor, fighting the darkspawn is something we can teach, the rest must come with yourself". It also makes sense that Duncan searchs for recruits outside ferelden, because you know the whole order was expeled and just came back thing, he also could use the dwarf stone sense ability as an asset to have (the guard in ostagar reports that all other wardens in the kings camp are HUMAN), so the only non-human is the PC.
Human Noble: Duncan already got ser Jory from Highever city, so now he went to the castle for another similar warrior-like recruit? Then he goes and try to recruit the teyrn´s son, even when grey wardens are supposed to avoid harassing nobles for their own good. He settles up for Jory, but he never really sees if Jory is any good first hand. Neither he sees that the Cousland is any good firsthand, he just goes by rumour because you won some tournament.
All in all, Duncan already has:
- Several wardens in the king camp, which all are human. They could be rogues, mages, and warriors.
- Alistair a warden already, who is a warrior (templar), so he is useful when dealing with emissaries but also in case a mage warden falls into abomination. It is useful to have him around in the wardens.
- Jory a recruit who is a warrior
- Daveth a recruit who is a rogue (lorewise more of a dagger user, ingame an archer)
- the PC who is a recruit as well. If Duncan already has one warden warrior in Alistair, plus a warrior-recruit and a rogue-recruit, it makes sense that he would look for:
1. Anything but a warrior. So he should look for a mage or a rogue.
2. Something with skills that humans do not have, so he could have went to the dalish to find a great archer, or to the dwarves to get a warden that can guide them through the deep roads easily due to stone sense.
As far as i am concerned, the ones that make most sense to me, in this order, are: mage, dwarf casteless and city elf.