The cation given in question is allylic carbocation. So the cations which are benzylic and aromatic will be more stable.
(i) is benzylic cation, hence more stable.
(ii) and (ii) are aromatic compounds, hence will be more stable.
(iv) is 2o carbocation stabilised by +I and hyperconjugation only. So less stable.
(v) is benzylic carbocation and more stable.
(vi) is stabilised by dancing resonance and is more stable.
(vii) is unstable since +ve charge is present on the alkene.
(viii) is stabilised by resonance and is more stable.
Therefore, 6 cations given here are more stable than H2C=CH−⊕CH2.