well it depends,
Cockatiels are great! Friendly sweet, not very nippy/bitey. A bit to dusty and dirty for my taste.
My experience with Caiques is a lot different from the other poster, my friend has one who is 5 blackheaded I believe who just loves everyone, into everything and a fast little bugger.
Eclectus is what I have, I have a little boy and Im a bit biased. They are more work than say a cockatiel, but the lack of dust definitely won me over. The diet might be a bit tricky though for some people.
oh Linnies! Lineolated parakeets may be small but they are the sweetest little guys, people really tend to over look them, I dont understand why they are not very popular.
Have you looked at different types of conures? Ill be honest though, the louder ones like the sun conures drive me crazy with that high pitched shriek, I like green cheeks and the quieter ones.
A word to the wise though, getting a bird from a breeder who well socializes them makes all the difference in the world. How can you expect say a bird who has been raised in say PETCO to know how to live in a family situation. They have never been taught to live in a home with a family.
I would really look for a breeder like this:
Pamperedpeeps Aviary-* Eclectus, Solomon Island Eclectus
where the babies are raised with the family and are well socialized