Which of the following compound will most easily be attacked by an electrophile? 

Electrophiles are electron-deficient species and preferentially attack aromatic rings with higher electron density. Therefore, the compound that donates the greatest electron density to the benzene ring will undergo electrophilic substitution most readily.

Benzene serves as the reference compound and is neither activated nor deactivated.

In toluene, the $$-CH_3$$ group increases electron density through hyperconjugation and the $$+I$$ effect, making the ring more reactive than benzene.

In chlorobenzene, chlorine exerts a strong $$-I$$ effect due to its high electronegativity. Although chlorine can donate electron density through resonance, the inductive withdrawal predominates, resulting in overall deactivation of the ring.

In phenol, the oxygen atom possesses lone pairs that are donated into the aromatic ring through a strong $$+R$$ effect. This resonance donation significantly increases the electron density of the ring. The resonance effect is much stronger than the opposing inductive withdrawal of oxygen.

As a result, phenol is the most electron-rich compound among the given options and undergoes electrophilic aromatic substitution most readily.

Hence, the correct answer is:

$$\text{Phenol}$$