For 5th generation I would have to argue that psychic, fighting, steel, dragon, and water are the best types for various reasons. Although the polls done here have few voters I'd say that they are pretty accurate. However right now I’m going to talk about the type that has shined quite bright this generation, which is fighting.
Fighting- Fighting types this generation are all about brute force. The Pokemon that have this type of style in OU would be Terrakion, Conkeldurr, Mienshao, Infernape, Virizion, and Breloom. Each of these pokemon have different characteristics that help their illuminate their viability like: Terrakion's Rock type in sand, Conkeldurr's bulk, Mienshao's regenerate ability, Infernapes large group of powerful moves and versatility (Close Combat, Fireblast, Flare Blitz, and even Stone Edge), and Breloom's poison heal ability + the move spore. Virizion isn't one of the most powerful fighting pokemon, but can not be over looked. Virizion has a great typing of Grass/ Fighting base 129 special defense, base 90 attack, base 90 special attack, the ability to boost it's attack, special attack, and special defense (some at the same time), can heal partially with a stabbed giga drain, and learns powerful moves like FocusBlast, Close Combat, LeafBlade, and Stone Edge. All of this and Virizion is a pokemon that isn't often countered in a large majority of teams.
Each of these pokemon have a base attack, special attack, or both of 90 or higher. This partnered with their excellent typings and stabs makes them popular in use enough.
With each boost along with their sheer strength fighting Pokemon are truly a force to be reckoned with this generation especially, because of the amount of new fighting Pokemon and the boost on moves like Hi Jump Kick and Drain Punch. The extent of these Pokemon's viability and strengths don't end with these facts alone. These however are just the major factors that make fighting Pokemon so exceptional in this generation.