Most common causative organism for lobar pneumonia is -
Streptococcus pneumoniae is the Most common causative organism for lobar pneumonia
Adult: MC serotype associated: Type 1-8
Children: Type 6, 14, 19, 23 are frequently associated serotypes
Most Virulent: Type 3.

Ref: Apurba Sastry’s Essentials of Medical Microbiology 1/e p233 , Ananthanarayan 9/e p222
Staphylococcus aureus
Streptococcus pyogenes
Streptococcus pneumoniae
Haemophilus influenzae