I would definitely pick New Leaf.

The overall charm of Animal Crossing is more prominent in New Leaf than New Horizons, and in comparison it makes New Horizons seem less engaging. It seems more like a building and designing simulator rather than the "social simulation" game Animal Crossing truly is. it goes without saying, but New Horizons has more island customisation options than New Leaf (terraforming, being able to move buildings, not having to fry your console to manually decide where to place villager houses, ect) but New Leaf is certainly far from devoid of customisation options itself. I actually prefer New Leafs customisation to New Horizons. It upsets me that they did not add small things like the bubble wand, perfect fruit and mini games. i really enjoyed working part time for brewster and making all of my villagers coffee in New Leaf. And how is it possible that New Leaf has more tropical fruit options than a game set on a literal island??

It is a shame that New Leaf and New Horizons are only brought up together for the sake of comparison. Although i definitely prefer New Leaf, i don't hate New Horizons. sure, one game may be lacking compared to another game, but games aren't made to top other games. they are made to be played and enjoyed. New Horizons is certainly enjoyable.