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

Cationic polymerisation needs a monomer whose substituents stabilise the carbocation; two methyl groups in 2-methylpropene do this. Acrylonitrile and methyl methacrylate carry electron-withdrawing groups and suit anionic polymerisation.