The question may seem easy but damn, that's actually tough to choose just 1 update. I've first heard about Minecraft when it was about to leave Beta version and got the game somewhere around 1.5-1.6. Huge amount of things changed since that days, a lot of great updates appeared, both from the Notch and Mojang.
If I would have to pick ma favorites, then they would be 1.13, 1.16 and Caves & Cliffs. And by saying Caves & Cliffs, I'm talking about whole expansion, from 1.17 up to 1.19. For me, it counts as 1 huge update split up into 3 parts.
And that's what I'm going to choose, the Caves & Cliffs update. I love technically everything we've got, starting from new world high, ending on AMAZING world generation with new wonderful caves and, well, cliffs 😄
The new ores looks fresh yet I still love to see old-looking diamonds on the YouTube videos, it just feels so nostalgic. With new cave biomes the underground finally became to be interesting, rather than just travelling through the endless maze of stone corriders, you can travel through endless maze of stone corridors with dripstone 😅
Jokes aside, the giant deepslate caves filled with lava at the bottom and lush caves flora atthe ceiling is something really cool to watch,
And the game finally got a bit horror style, different from just looking behind if the Herobrine is not watching on me in the 1 block thick and 2 blocks tall tunnel 😫 The Warden is the only mob that I fear to encounter but also the only one I'm so fascinated about just how unique he is. I've literally never used my firework elytra quicker than when he started to dig up from the underground.
Damn that post is long... Okay so that's my (shortened) thougts about my fav. update. Can't wait to read your posts too, bye!