Most of my favorites are from the end of S5. One moment that always makes me smile is kind of strange. When Michael sees Brian and Justin's wedding invitation and he starts getting upset, Ben comes over to the table with this big smile and says "Apple cinnamon pancakes" very loudly to distract Michael. For some reason I just love that.
Along that same vein, everyone's reactions to the invitations are fabulous. That may be one of my favorite moments of the entire series. The hot sex doesn't hurt either:)
My heart just stops when Brian says to Justin "I was so fucking scared" it gets me just as much as the "I love you."
Justin's confrontation with Craig in his office. It breaks my heart, but the dynamics between Justin and his family were some of the best parts of S1 and I was so glad to see the series come back to that. Justin really needed that moment to let go of Craig.
I love Brian's first proposal because it is so random and out of character which is what makes it feels so genuine to me.
The look on Brian's face when Justin says yes and they hug kills me.