The most widely accepted explanation for motion sickness (which includes Airsickness, seasickness, carsickness, simulatorsickness and since recently selfdriving car sickness) is the sensory conflict theory. This theory states that if you sense a different motion via your vestibular system (the inner ear), then what you see via your visual system (your eyes) which does not align with your experience then you will encounter motion sickness. It is important to notice that this mismatch is matched with your personal experiences of motion, therefore you can get acustomed to certain types of motion after having experienced them a couple of time. In other words: You learn to accept a given motion, which then will not make you motion sick anymore (or in some cases, not as much). Additionaly, people are especially suceptible to certain frequencies of motion. The Standard ISO-2631 (unfortunately restricted access) states that 0.2 Hz oscillations are especially nauseating.
Given this background, you will have to identify a maneuver which results in the biggest motion conflict possible and ideally features a lot of motion in the 0.2 Hz range. Because there are currently only very unsufficient models of this motion conflict (which is an ongoing research topic, one model is described here for example) it is imposible to classify which flight maneuvers are best suited for this task (or at least with good accuracy). I would propose motions with unexpected centrifugal forces, and unexpected orientation turns. Centrifugal forces screw up the correct sensing of Earth's G-Vector by the vestibular system which then is interpreted (by a lot of people) as a unexpected motion. Because of a lack of accurate models it is not possible to foresee which maneuver is the most nauseating, but I would propose a stall turn (or hammerhead) as it ticks all these boxes.
However when going back to the ISO-2631 I can speak from personal experience that the absolute fastest way to make people airsick is by flying a vertical standing sinusoid flight-path with a frequency of 0.2 Hz. This is an incredibly fast way of making people airsick.
One note given your sentence:
Likewise, a 1G barrel roll is much less disorienting than extended inverted flight.
Motion sickness research suggests that this is not true, simply because the human motion sickness mechanism is centered around oscillation in some sort. Extended inverted flight is a static condition while a 1G barrel roll is a motion with a distinct frequency spectrum, which would be more adapt in provoquing motion sickness...