What suits best for lateral controllability at high Angle of Attacks- a vortex generator or wing slots? At high angle of attacks, the prominent problem is the separation of airflow. And if this airflow separation is near ailerons, we lose lateral controllability.
And the solution is making the flow turbulent.
By adding slots in a wing at correct positions, we are capable of re-energising the flow. It is the same thing a vortex generator does (I presume).
But what device is best suited for the current case?
From an unknown source, I found:

At high angles of attack, the airflow over the ailerons can be separated from the surface. Vortex generators, positioned just forward of the ailerons, are designed to re-energise the boundary layer and help to prevent this separation. This makes the ailerons more effective at high angles of attack

(I do not how effective this reason is.)
And therefore, please consider the case of Leading Edge Slats (which creates a slot) too. At a high angle of attack, a LE slat helps by re-energising too. And If we have a turbulent flow (by employing LE slats) in the early stage, (I think) it is better than the case where we put vortex generators near ailerons, which may encounter an already separated flow.
Edit: The aspect of lateral controllability dictates the conditions where one wing can be at higher AOA than the other. Will Slats not fail to provide the desired effect due to its symmetrical effect (increasing CLmax simultaneously for both and stalling one of the wings and making into a spin)?
Does the &quot;symmetrical effect&quot; apply to VG and slots near the aileron too?
Vortex generators can be used at the leading edge to energize flow and delay separation.  They are widely used as an add-on device to improve low speed behaviour of a range of light aircraft, and in that usage they have a similar effect to a slot or slat in that they both increase stalling AOA and improve aileron response in the stall. I call them &quot;poor man's slats&quot;.
However, slats are able to energize the boundary layer to significantly higher angles than leading edge vortex generators.  As an add-on, VGs are great, but if your objective was to maximize AOA at stall, you would go with a slot or slat.  A VG might raise stalling AOA from 15 to 19 or 20 degrees, whereas a slat/slot is good to the mid 20s.  Similarly, a VG can increase a wing's Clmax by maybe .3 to .5, whereas a slat/slot adds about 1.
To answer the question specifically, for maximum benefit for lateral controllabilty at the highest AOA possible, you would use a slot/slat. VGs would be a second choice, unless your objectives were more modest, in which case if they do the job, they would be a much cheaper and simpler solution.
There was a period back in the 40s when leading edge slots in front of the ailerons were popular as a way to maintain aileron authority through the stall (Globe Swift, Stinson, etc). Later, designers just included sufficient wing washout to achieve the same goal without the manufacturing hassle and drag, and fixed slots disappeared.
Modern transport airplanes and specialty STOL airplanes will use slats across the full span to increase stalling AOA and maximum lift the wing can make, as opposed to just improving aileron control.  Same with leading edge VGs, although the benefit is only about half.  On light twin engine airplanes, VGs have miraculous benefits for single engine performance.