Normal saline is 0.9 N NaCl (salt water that is isotonic with blood).
Lactated Ringers is very similar to normal saline but has lactate, potassium and calcium added as well. Ringers is the same as Lactated Ringers minus the lactate.
D50 is 50% dextrose (sugar) that is isotonic with blood.
I almost always use NS (normal saline). It is most compatible with medications and blood transfusions. If a patient requires a large volume of fluids (like a burn victim), then LR (lactated ringers) is better because high volume NS can lead to a hyperchloremic acidosis which increases death rates.