Kicking Sabaoth's ass and obtaining the BFG 9000
Sabaoth (aka demonically possessed Sergeant Kelly) can be killed easily in less than 10 seconds with 5 rockets and a fully charged Soul Cube. Killing him will give you Campbell's BFG 9000, which is an awesome weapon you will be putting to good use for the rest of the game.
Also, I spend more time admiring the BFG 9000 and watching the briefing video than actually fighting Sabaoth himself