To illustrate just how powerful it is, Water Spout from Choice Specs Kyogre is capable of 2HKOing Eviolite Chansey the most durable special wall in the game.
While at full health and with its Drizzle effect up and running, Kyogre's Water Spout effectively has an absolutely astounding Base Power of 337, backed up by a massive base 150 Special Attack stat. The most blatantly obvious factor is the fact that Kyogre has access to the absolutely strongest move in the game (barring a Flash Fire-boosted Eruption from Choice Specs Quiet Heatran in the sun): Water Spout. Drizzle goes a long way in making Kyogre the cream of the crop, but there are a ton of of traits that make it the monster it is. Consequently, expect to face rain every other match. Kyogre has pretty much always been the king of Ubers, garnering over 50% usage in both DPP and BW.
Many people will opt to stick with the much more dangerous Ubers who benefit from weather, but don't overlook the underrated threats the metagame has to offer, or you could find yourself on the wrong end of a sweep from one of them. Ubers has long been a tier where Pokemon can fill niche roles alongside the most dangerous Pokemon in the game, and weather amplifies this to an amazing extent. Rain has always been the most popular weather in Ubers due to Kyogre's dominance, but sun and even sand are still very viable. Ubers has been home to weather wars since the release of Ruby and Sapphire and their respective titular legendaries, and the struggle to gain control over the environment can be just as titanic as the inducers. Many people are calling BW the weather generation, but it is not the first time that weather has had a drastic effect on a metagame.