OK... Step away from the meat counter... Now take a deep breath...
Spare ribs and Baby Back (Loin Back) ribs are both pork ribs. Beef ribs are, well, beef.
You will also see Beef Short Ribs, which are still beef (doh!) but from a different part of the cow.
Which is best? That's like asking a mother to pick her favorite child
All of them are good, but they are different. Spare ribs are typically meatier than baby backs and have more of a "pork" flavor. But, they have more fat and cartilage than baby backs so some folks don't like them. Baby backs are considered "better" than spare ribs mainly, I think, because of marketing. Restaurants advertise their baby back ribs all of the time, so folks think they are better. Personally, i prefer a well cooked spare rib any day. Spares typically take longer to cook than baby backs.
Beef ribs are completely different since they are beef. The most common beef ribs for BBQ are the long bone ribs that are part of the rib roast. They are hard to find with much meat on them because the meat can be sold at much higher prices when it is part of the rib roast. If you do find meaty ones that are great. Because of their size, they take longer to cook than either of the pork ribs.
The other beef rib that is commonly available it the beef short rib. These are most commonly braised, but they can be cooked low and slow with some success.
Does that help?