Fluttershy and Pinkie Pie.
Reasons being:
Fluttershy is very shy, and doesn't get out much. I believe Pie helped her somewhat get out of her shell, but I think since she didn't have friends before, she developed imaginary friends before she had real ones, and kept them. Her friends are mostly woodland critters though.
Now Pie, it seems to me she creates random friends out of thin air if she feels lonely. Like for example, She created a turnip, and a sack of flour for friends, and gave them names and personalities. I think if it's late at night, and she's lonely, she'd probably create some to keep herself company. She uses imaginary friends as a comfort item of some sort, which I believe is really, really, really smart.
Twilight however, like people were saying, really doesn't have a lot of friends, but she didn't want them before because she had her books.
I honestly feel like Twilight doesn't really need anypony else but her mane friends.
That's my opinion.