"Doppelgangland" and "The Wish". Me, a sucker for Vampire Willow ? Noooooooo...
But I also voted for "Anne", "Earshot", "The Prom" and most importantly "Revelations"... not because I watched them the most intentionnally, but because, when I learnt the existence of BtVS (I was like 9 or 10), season 3 was on air. And I remember pretty vividly having watched these episodes, totally afraid of the HORRIBLE MONSTERS! (I also watched Charmed cos it was less hardcore...) And I'm pretty sure that, in the years to come, every time I watched TV and Buffy was on, it was one of these episodes. Especially "Revelations". I ALWAYS fell on that one, and I've always remember the Willow quote "I wanted you to be the first to know that... There's a demon behind you.". God, even now I remember the French version of it, though I haven't watched it in French since the first time I watched all the show. (And that was because only French was available)
"Je voulais que tu sois la première à savoir que... Il y a un monstre derrière toi." Oh God I already loved Willow!