Commonest cause of heart failure in infancy is –
Congestive heart failure 

Congestive heart failure is state in which the heart cannot produce the cardiac out required to sustain the metabolic needs of the body, without evoking certain compensatory mechanism.
The most common cause of CHF in infants —> Congenital heart disease.
The most common cause of CHF in the older children —> Rheumatic fever & rheumatic heart disease.
Myocarditis
Rheumatic fever
Cardiomyopathy
Congenital heart disease