Physically: Illyria
Emotionally: Faith (yes, you read that right) Your all probably thinking I'm crazy, and I can see were your coming from. Yea, Faith broke down and went all evil, but she came back from that and kicks ass as a Slayer now. She's had a lot to deal with in her life, but she's gotten through it without much help (save Angel). Buffy on the other hand has always had a stable up bringing with emotional support from friends and family, put her in Faith's situation and I dont think she;d do so well.