Which is the lest severe from of heat illness

Heat exhaustion is the answer

Heat stroke you would go through FAST which means Face, Arms, Speech, Time. So all of them are pretty severe.

If you're talking about Heat exhaustion the signs and symptoms are headaches, sick to your stomach, some may pass out, and they don't require medical attention right away. In the case of exhaustion you'd get them to shade and give them water and cool them down.

Hope this helps