The Spectacular Fog Scene
I'm surprised no one on this board has brought up what is -to my mind- the single best scene in the film: the fog engulfing Giuliana's friends on the waterfront after they've left the loveshack. Here's a link to it.
http://www.youtube.com/watch?v=wLWGXm-rM4Y
The stillness and formality of the groups composition is beautiful and extremely eerie. As they all stare fixedly at Giuliana you realize how palpable her isolation really is, and I believe this truly hits her as well (as the lady states later, "we are all separate." Prompting her rushed borderline car-off-the-pier suicide attempt). As the figures are enveloped one by one in the mist a shiver ran down my spine. This was a moment of pure cinema, one which works on a visceral, intellectual and thematic level. Hella cool.
I can think of only one other transcendent scene like this done by Antonioni and that's in The Passenger's penultimate tracking shot. Everything comes together so beautifully both technically and in narrative terms there. Any others you can think of? Also, feel free to say I'm wrong or crazy misjudging it, but please explain why.