This pokemon has been top OU in every gen (if it wasnt because DP it would be snorlax).
In GSC it was the best pursuiter, and a good curser and mixed sweeper.
In ADV it was the best mon. It could be one of the best hard hitters (cb), or the best late game sweeper (dd), or a great wall breaker (sub sets), or a great supporter pursuiting ghosts and it can also easily lure the counters of its main sets. It also had sandstream, something that was very helpfull for stall teams.
In DP its one of the best mons in this metagame (top 3 at least). Scarf is everywhere, so stalls can trap the most common ghost (rotom-a) and the most common spinner (starm). CB is still a monster that can 2hko everything, and DD is still a very dangerous late game threat.