Intuitive gameflow. Don't make me waste clicks and mouse movement.
Yeah, not a game mechanic as such, but a question for mechanics is too broad. If you would limit the genre at least. My favorite mechanics in a deck builder are different from my favorite mechanics in a rpg and different from my favorite mechanics in a puzzle game.
Cards: do not lock the deck while displaying animations. That gets boring after 1 minute.
Reading: do not make me click to see the full text.
In general, the interface is more important for me. Like adjustable key bindings and quality of life functions. The actual game mechanics usually are what type he game is. So saying that this and that is my favorite mechanic would just be like saying I like a genre.
Sooo, deckbuilding, I guess would be an answer to your question ;-) And in a way, equiping your character in any rpg is also a sort of equipment loadout building. It is the optimisation aspect, and that is a core aspect of gaming, which also gives game theory it's name. Though that is more math than fun.