Although the scenes with Kelso donating his 'little soldiers' were more heavily laden with a series of hilarious and classic one liners("roll up my sleeves, get a little sweaty and earn some extra cash" and "oh, to be a working stiff" are some of my favorites), this was not the storyline with my favorite scene.
That honor goes to the infamous slow motion 'He Was Drugged' scene:
Red tells Eric he loves him, and we eventually find out that the dentist doped him with knockout juice to repair a cavity, but before Kitty can warn Eric, he says it back to a 'sober' Red, and everything from Kitty's "NOOO!!!" to the final part where Red says "Go to your room" is non stop hilarity. The jello falling to the floor, the priceless facial expressions, the realization, everything was just perfect, I don't think I ever laughed so loud and hard in my life.
If you're a long time 70's Show fan, watch this episode, because the more you know of the 4 years of history that precede it, the more the scene will just click.
Also, this episode highlights, IMO, Fez's peak moment of candy addiction as he continuously eats the valentine chocolates that he's supposed to give to big Rhonda, it's not quite up there with his porn addiction episode in season 7, but it's still solid laughs.
So there you have it, an A plot with numerous one line home runs and a B plot with an infamous slow motion scene make this my favorite episode in 70's show history. Although I'm sure episodes such as the Grage Sale and Water Tower will be the sympathetic favorites (Although both great episodes mind you) this will always be a sentimental fave of mine.