Phenol is least acidic due to non equivalent resonance structures in which negative charge is delocalised over one oxygen atom and less electronegative carbon atom.
Carboxylic acids are the most acidic compounds among the organic compounds.
Benzoic acid is more acidic than acetic acid due to resonance effect in benzoic acid.
Chloroacetic acid is more acidic than benzoic acid due to -I effect of chloro group.
The decreasing order of acid strength is
Chloroacetic acid > benzoic acid > acetic acid > Phenol