Most reactive towards $S{N}^1$ mech

(1) & (2) are least reactive 

Aryl halides & vinyl halides are least reactive towards nucleophilic substitution $\left(S{N}^1\left(\text{or}\right)S{N}^2\right)$

Reasons for non-reactivity of Aryl and vinyl halides 

(1) Stabilize through resonance 

(2) $C{H}_2=\stackrel{+}{C}H$ (or) $\stackrel{+}{\mathrm{ph}}$  cannot stabilize through resonance

(3) C - X bond acquires double bond character due to resonance

(4) Difficult to remove $X^{-}$ from more EN $s{p}^2-$carbon

(5) $C{H}_2=CH-$group (or) $C_6{H}_5-$group itself is electron rich

Rate of  $S{N}^1\propto$stability of carbocation 

stability of $C{H}_3\stackrel{+}{C}{H}_2>\stackrel{+}{C}{H}_3$

explained on the basis of hyper conjugation and inductive effect