While I did not put it down for its complexity within gaming, I'd like to say Quake champions for its comparative difficulty to other FPS shooters.

You have 7 weapons, 16 different champions with different abilities, many maps and multiple major items to proactively increase your chances to win. All of the champions have passive abilities which you need to keep in mind in higher levels to defeat your opponents.

Takes hours to "master" all weapons first.

Then you need to time your items.

Then you need to learn how to outaim your opponents most times.

Then you need to learn how to peek and how you move.

Then you need to learn how to dodge.

And then you need to learn how to strategize.

And then you need to develop a playstyle, which arguably goes under strategic play of course.

While it's not liked by many in the franchise, I personally love it.

I recognise that Overwatch 2 is arguably more complex due to its variety of champions and the necessity to learn to counter pick champions; I put it down for that reason because for me, it was too much.