Who is good to recruit for the Blades?

Aren't the Blades traditionally all Breten?


???

All Breton? No, they're generally any race. They were originally Akavir. In Oblivion, you had Imperials, Redguards, etc..

Anyways, I'd suggest recruiting followers whom you'd otherwise pay for. Marcurio, for example. After that, they're free. That, and maybe Lydia, just to get her out of your house. Weapon/Shield characters like her probably make the best use of the gear they get.

I usually recruit Mjoll 1st and after that I'm not sure who.

http://www.uesp.net/wiki/Skyrim:Followers

There's a list of followers and which can and cannot join the Blades.
Personally, I've thrown in two housecarls (Lydia and Jordis) and probably with throw in Annekke.

I like to recruit Erandur (Dawnstar) because of his backstory, and because of his decent mix of skills.

The best choices are followers that you run into on a regular basis and do not want to. This is the perfect way to get Housecarls out of your house without having to kill/sacrifice them. Goodbye, Lydia.