Which types of reactions are easiest to establish through classical conditioning?

Classical conditioning is easiest to establish for involuntary reactions mediated by the autonomic nervous system.1

These responses are reflexively elicited by specific stimuli and cannot be modified by their consequences, unlike voluntary behaviors. Examples of such involuntary responses include glandular secretions, such as salivation, as well as other physiological reactions like blushing or the release of adrenaline in response to fear or anger. Because these reactions are not under voluntary control, they are particularly well-suited to the process of classical conditioning, where a neutral stimulus becomes associated with an unconditioned stimulus to eventually elicit the same response.21