I voted for lessons, conversations with dead people and chosen.
Lessons because it was the first episode of Buffy that I ever watched when I was young and therefore it has a nostalgia quality attached to it. I rewatched the episode recently and I think its a great season opener! The relationship between buffy and Dawn is adorable and its so cute how protective Buffy is. I liked seeing the upgraded Sunnydale High.
Conversations with Dead People, its quite a sad episode for me when Joyce appears. I think it might have brought a tear to my eye once or twice lol. I love the bit with Cassie and she tells willow about Tara. A close second on the list if it wasn't for chosen would probably have been Help.
Chosen because of course its the finale. The final battle is iconic. Another one that gets the feels with Spike at the end when Buffy says ‘I love you’ and then Spikes ‘No you dont but thanks for saying it’