Specifically with regards to Vanilla WoW, it might be interesting to dive into what you mean by this.
Here’s my gut feeling, if I had to guess, and I wouldn’t take that guess to the bank. I would maybe set up a test based on what we talked about above - rotational complexity on Patchwerk for optimal melee dps output for example - or dive into the WoWSims github codebase and try to understand Feral vs Warrior sims for Vanilla to get something we can actually measure… but if I had to guess:
I feel that Vanilla warrior in PVE is probably the most complex class to play in a PVE raid setting. It certainly is the class among those that I personally play (which includes druid) where I require the most macros, and have spent the most time learning and making macros, specifically for Vanilla warriors.
As an anecdotal aside, and obviously more relevant to tanking (which in my opinion is a regular part of playing either of the two classes) we can have a look at Elite’s Low Level Warrior tanking guide, aimed at players new to Vanilla Warrior and tanking at lower levels, as an introduction to the class:
Specifically, I’d look at the rotational and stance components and compare that to what someone might say about Bear tanking, which would be something like, “Stay in bear stance. Use Maul and Swipe if/when you have enough rage. Use Demoralizing Roar to keep the debuff up on mobs.”