Define "most difficult." Are you talking about most mentally challenging? Or busiest? Some cases are technically very easy, but take so long (i.e. some back cases) that after a while, they get difficult because they are so boring. And any case can suddenly become busy, difficult and challenging, all at once. Increases the sphincter factor by at least 9 degrees.
I think generally the two busiest cases you can do, in order, are first, carotid endartarectomies, then open heart procedures. Carotids, by the very nature of their disease process, are hypertensive. The blood flow to the brain is reduced by a stenosed carotid artery, and higher pressures are needed to maintain an adequate flow. Unfortunately, everything we do to anesthetize the patient works to lower blood pressure. So, it's possible to be constantly adjusting different factors to maintain an appropriate depth of anesthesia, while maintaining adequate blood pressure to keep the brain alive and healthy.
Open hearts are challenging because no two are the same. No one coming off cardiopulmonary bypass is EVER stable. The question is, how will that instability manifest? Arrhythmias? Blood pressure problems? Strokes? Asystole? All at the same time? Can be a very busy time.
Craniotomies, particularly cranis to clip aneurysms, can also be sphincter tighteners. Lots of reasons for that, but you have to be on your toes. Still, I think they are my all time favorite cases to do anesthesia on.
I never define a case as bread and butter, or easy, because those are the cases that can bite you hard. Let you guard down, and that patient will do something totally unexpected.