Can you please tell me which is the top cold med for a18 month old baby?

Not much : Most cold medications are not appropriate for young children and cough meds should definitely not be used in children this young. Most infections children get are viral and do not require any antibiotics. Most coughing is caused by dripping down from the nasal passages. Nasal saline and suction can help and your medical provider can provide guidance for other meds that may provide benefit.