Which monomer is best suited to cationic polymerisation with $\mathrm{BF}{}_3$ and a trace of water?

Cationic chain growth needs electron-donating groups that stabilise the carbocation; two methyl groups give a tertiary carbocation. $-\mathrm{CN}$, $-\mathrm{NO}{}_2$ and fluorine substituents do not stabilise a carbocation.