Apart from my all time favorite QaF-moment (when Brian, at his most open and vulnerable, is looking at Justin, then dives in for the kiss like he's drowning, and Justin catches him), there is this scene when Brian comes over to Michael's house and berates him on corrupting Justin. I love the raw emotions of this scene. Also, the little whimpering noises Justin makes in response to Brian saying ILY. And I'll always cherish the one thing that surprised me the most, because I wasn't spoiled for that: Blake and Ted uniting.
OK, that's just me scratching on the surface of all the love that was in S5.