For me it's definitely Buffy.
It's my favourite show and it's also a show that put me thru the emotional wringer on so many occasions .
There's so many moments there id love to see again for the first time and the start of every season was always so exciting because the status quo was often changed to something really adventurous and thought provoking.
I can still watch it now, but the emotional impact is so great first time round that the effect is not the same on rewatching.
Contrast that to The Wire, my second favourite show of all time. I am on my 5th watch of The Wire now and this is a show that gets better with each watch. It's biggest strength was not the big moments, although it had its fair share, but the consistent quality of its writing and characters and the believable world that I feel I shared with them (Buffy has a bit of this also, clearly).
Deep Space Nine would possibly be my 3rd favourite, and again i don't need to wipe the slate clean to watch it again. I'm due a rewatch soon and I don't think it will be diminished any by knowing what is going to happen.
So, Buffy for me, more tears shed and heart strings tugged during that than any other - and I'd do it all again!