I've got to go with "Seeing Red". The grisly fate of that poor defenceless pizza slice IF aside, this is a really charming and heartfelt little episode, featuring the (regrettably) only appearence of one of my very favourite Foster's characters. Red rules, in no small way because he clearly has the spirit of the 1960s in his heart.
And it's funny too. It was among the first episodes I ever saw, and one of Terrence's lines provided me with my first real belly laugh from the series - "Your name can be GREEN...no, RED!"
As a Wilt fan, I feel somewhat obliged to give "Where there's a Wilt, there's a Way" a couple of points for being the first truly Wilt-centric episode, but I wasn't satisfied with the ending, which had the poor guy endure hell in exchange for nothing in return. I think my second favourite would be "Sight for Sore Eyes". And "Phone Home" is a good one. I was genuinely surprised to discover just how many people here at NF seem to hate it. Each to their own, I suppose.