Paige's nasty treatment of Cole
Prue and Cole was always an interesting relationship to me. She correctly suspected that something was wrong about him. After he comes back, she's cold toward him again, but then they started to slowly like each other and made one hell of a team. The Western and Death episodes being some of my favorites.
When Cole is forced/brainwashed to kill that witch by that other demon in order to get his father's soul back (I really wish they further explored Cole's relationship with his father), you could see a genuine sadness on Prue's face mixed with anger and disappointment, along with Piper and Phoebe too. That was Cole and Prue's last scene together which makes it even more sad to me.
However with Paige, she turns on Cole for no reason, long before that Source/Cole possession nonsense started. She sees Balthazar in the book and immediately judges him, making unnecessary snide remarks.
The irony is Cole was the reason Paige became a Charmed one as quickly as she did. Piper was too busy mourning Prue to care, Leo had no idea who Paige was, Phoebe wanted to help but again didn't know who she was. But it was Cole who found out from his Underworld contacts, motivating Piper to summon Patty and Grams to find out the truth, thus leading to the Charmed ones being reborn.