What suits best for lateral controllability at high Angle of Attacks- a vortex generator or wing slots?
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.