Which king of India opposed Alexander the Great?

As to if King Porus opposed him successfully is all up for debate. At the Battle of the Hydaspes the two armies would confront each other at the Hydaspes river in 326 BC. This occurred shortly after Alexander had finished off the Achaemenid Persian Empire and decided to head south in order to conquer India and reach what he thought would be "the end of the world", thus ending his campaign to become the "king of all of Asia". The interesting thing here is that the only person who truly seemed as if they wanted to go any further on this now nearly 7 year campaign was Alexander himself. His army was tired and wished to go home as their main goal, ending the Achaemenids, was completed. Nonetheless, reluctantly, they headed south under their generals orders to conquer India.

At the river Alexander would outnumber the Indians, and with both his superior numbers and use of flanking maneuvers to cross the river at night he would have them defeated. The story goes that Porus, after being captured by Alexander's men, was asked by Alexander how he wished to be treated. He replied: "Treat me as a king would treat another king". This reply impressed Alexander and Porus was made the new Satrap of Punjab, thus allowing him to remain in control over his lands.

The interesting thing here is the question, who really won in the end? After the battle the Macedonian army was so frightened by the prospect of fighting another large Indian army that they threatened to mutiny if Alexander didn't take them back home. It's fair to say that if Porus hadn't shown up and opposed Alexander that the Macedonians could have continued their march south without any fears what so ever and before they would know it there would be no turning back. But as for that question, I will let you decide. Hope my answer helped!