depends on what you are looking for.
gall bladder is exception because it's almost exclusively evaluated by ultrasound because its a hollow organ, relatively superficial so it shows up well on this modality
heart--we use mainly ultrasound (echo)
brain-mainly ct and mri
kidney-us for looking at parenchyma-ct for looking for renal stones
liver-ultrasound to look for gross abnormalities, fatty liver, biliary system, etc. MRI or CT to look for definite masses
uterus-best evaluated by ultrasound
bladder, ultrasound or cystoscopy
lungs: CT scan generally
spleen: ultrasound or ct