Willow and Anya
Willow Rosenberg had her first encounter with Anya in "Doppelgangland," when Anya approached her asking for help with a spell. Anya was desperate to get her vengeance demon powers back, after losing them in "The Wish," and Willow was desperate to prove that she can be a little dangerous. The spell went wrong, and they ended up conjuring Willow's vampire self from an alternate dimension (the dimension to which Anya sent Cordelia in "The Wish").
After "Doppelgangland," Willow and Anya's relationship has been up and down, switching from friends to rivals constantly. Despite their differences, which mostly centered around Xander, Willow and Anya usually fought for the same side (except for those brief moments where either of them did some evil things). They have both helped each other out, though not as readily as they would insult the other. On their best terms, they considered each other as a friend.