What's more sedating diazapam or mirtazapine ?

Diazepam: Hands down, used as a sedative for many years.

Diazepam: This is a long-acting anxiety medication whose sedative effects increase proportionately with dose increase. Metabolites may persist in the blood stream or tissues for more than 36 hours, so it has limited usefulness. Mirtazapine is an antidepressant, that may exhibit sedating effects, which may or may not be beneficial, depending on the patient. Diazepam should not be used for depression.