[doppler]

Out of the inductive and mesomeric effects, which is most predominant?

[Hunter2]

Depends on the substituent.

[spirochete]

When oxygen/nitrogen lone pairs are involved resonance tends to "win" over induction. For example oxocarbenium ions and iminium ions are both very stable cations. And esters/amides are both less electrophilic than ketones because of electron donation from the heteroatom lone pairs.

When halogens are involved, induction tends to "win." For example halogenated aromatic compounds are less nucleophilic than unsubstituted compounds because they lead to less stable cations in electrophilic aromatic substitution. And acyl halides are more electrophilic than ketones. In this case of fluorine this is observed because of its exceptional electronegativity. In the case of other halogens this is observed because of their poor pi donating ability with carbon and other second row elements stemming from bad orbital overlap.