Whether betting or gambling is the most addictive is a very interesting question, though the answer might surprise you. Addiction is an interesting phenomenon and it is not necessarily ascribable to the particular object of that addiction.
Some people are prone to be addicted to a variety of things and others are very resistant to it; the former are said to have an addictive personality. Most addictions are replacement activities where the addict is addicted to something that is lacking in the rest of their life.
People can become addicted to many things. There are chocolate addicts; people who are addicted to love; sex addicts and so forth. That does not mean that chocolate and so forth are addictive; it just means that some people with a particular personality trait are prone to become addicted to them.
Naturally some things are naturally addictive, and those are the things that create a physiological change in the bodies of the people who use them. Most class A drugs are physiologically addictive, as are cigarettes. Some other substances, for instance alcohol, are capable of causing physiological dependence, but most people are able to use them without becoming addicted; however when they are used by someone with an addictive personality, they can be very damaging and even fatal.
Giving up an addiction is net necessarily easy, though some people are able to do so with relative ease. Others will just continue to relapse into the addiction time and time again. For instance non-addictive personality types are able to simply give up cigarettes after decades of smoking simply by making the decision to do so.
Thus, addiction is not in itself caused by the object of the addiction. The vast majority of people are able to enjoy both betting and gambling without any danger whatsoever of becoming addicted to them, whilst a tiny minority may, for one reason or another, become addicted.
Under normal circumstances neither betting nor gambling are innately addictive (they do not bring about physiological changes); though if you are a person with a particularly addictive personality then you will need to remain aware of that fact and take a little more care.