It is quite impossible to pick just one single most powerful moment in a discography littered with, you guessed it, powerful moments. So here are a few of my choices in random order.
"The Moor": From beginning to end, this is just one solid 12 minute block of power/awesomeness +4. A perfect composition. Each part leading to the next is powerful, the lyrics are among Opeth's best, the clean parts are the shit, everything is just major smoking hardon, except the longish intro.
"Serenity Painted Death": The chorus is fucking powerful.
April Ethereal and When's brutal opening minutes are so full of awesome.
The blast-beat verses of The Lotus Eater. Very powerful.
And so I could probably continue until I came to the few Opeth-songs I don't like. Opeth itself is a powerful entity of mass erectio.n.