The first big kiss between Meliodas and Elizabeth in the anime happens in episode 24 of the first season, titled 'The Seven Deadly Sins, Once Again.' It's that moment right after the fight with Helbram, when she's leaving the tavern. He pulls her back and kisses her, and honestly? I was watching it with friends and we all gasped in unison. It’s framed as her memory being taken away, which makes the whole thing bittersweet, but the kiss itself is passionate and kind of a big payoff for the will-they-won't-they tension they'd been building.
I’d argue the more significant one for their relationship happens later, though. The 'real' kiss, the one that feels like a genuine turning point, is in episode 4 of the second season ('Revival of the Commandments'). That’s when Elizabeth confronts him about his past as the Demon King’s son, and she kisses him to stop him from leaving and to prove her feelings. It’s less of a surprise and more of a deliberate, emotional choice, which resonated with me a lot more. The timeline gets a bit fuzzy with all the curses and resets, but those two moments are the major landmarks for their physical affection in the anime.