The lion has been called the King of Beasts for centuries, due to its majestic appearance and the fact that it is at the top of the food chain. In more recent times, wild animals have become generally referred to as 'jungle animals', as a way of separating them from 'farm animals', say, or pets. Hence, King of the Jungle. Whilst it is true that the majority of lions live on the open savannahs of Africa, there is a small population of Asiatic lions in the Gir Forest of India - the term King of the Jungle might more appropriately be applied to these lions than those in Africa. However, personally I feel that the title King of Beasts is more fitting for the lion, and King of the Jungle for the tiger, since tigers live in jungles and are the largest of all cats.
As to it losing a fight to the animals you mention, that's debatable. A battle between a lion and a tiger could go either way. There are numerous records of fights between these species, since many people in the past have wanted the answer to this question and tried to settle it by cruelly pitting tigers and lions against each other. In some cases the tiger wins, in some cases the lion. The outcome of such a fight depends on the individuals involved - their age, size, sex, health, experience, temperament, etc. Tigers are larger and stronger, but size and strength are not the only determining factors - a large but timid tiger would be likely to lose to a small but aggressive lion, even though it was stronger. Lions generally have more fighting experience - they often clash with other prides, whereas tigers tend to avoid conflict whenever possible.
Lions are known to have killed Nile crocodiles when on land, though in water the croc would have the edge, and a lion would destroy a gorilla in a fight - the lion is much stronger and better armed.
My money would also be on the lion in a fight with a bear. Though some species, such as the brown bear and polar bear, can weigh more than lions, most of their weight comes from their large, heavy skeletons and considerable amount of body-fat. By contrast, a lion's skeleton is quite small given the size of the animal, and its body-fat is minimal - most of its weight comes from its enormous musculature. Lions have evolved to hunt prey considerably larger than they are - they have to be immensely strong in order to bring down animals like buffalo. They also have much larger canines than bears, and the shortened muzzle and large sagittal crest on the skull gives them more powerful jaws. Not to mention their eighteen razor-sharp claws (bears' claws are non-retractile, and therefore blunt from constant contact with the ground). Brown bears are largely vegetarian - though they will eat meat, it is usually carrion or small or young animals that are easy to catch and kill. Polar bears, though the most carnivorous of the bears, prey mainly on seals, which are considerably smaller than they are, meaning they do not require as much strength to kill them.
I used to be a zookeeper and have studied animals all my life.