The episode where Lily is a birthday brat. I didn't like her before that episode but in this one she became the definition of a bitch. She was really mean, up her own ass and I just wanted to punch her near the end. I don't care if she had a point that Ted kept inviting girls to their private special occasions, it was still really mean of her to be like that to both Ted and his date.
A real friend would support Ted, but so far she's criticized every single one of his girlfriends, did things in secret to get him to break up with them, and then become a complete birthday brat causing her to be incredibly horrible to Ted's date. And then Marshall followed her way and SHOUTED at Ted's date for accidentally getting the age wrong BESIDES MAKING THE BIRTHDAY CAKE FOR LILY. Marshall was also an ass in that episode.
It wasn't very funny the only thing in that episode that worked was Barney's pose for pictures and Robin trying to get him to mess up before the picture took.
But Lily and Marshall pissed me off so much in that episode that I actually skip it when I'm watching the Season 5 DVD episodes