Maybe not the most powerful, but the best from a balance, storytelling, AND power standpoint. This is a list of the best class combinations in the game. Want your ranger to cast fireball? You’re a wizard Harry!
Want a paladin that can be stealthy? Give him a few levels of Rogue. These rules allow for a player to combine multiple classes together to make characters that truly suit exactly what the player is going for. In the D&D5e there are advanced rules in the Player’s Handbook concerning the topic of multiclassing.