This is Buizel's primary sweeping set. With only a bit of support, this set can cut through unprepared teams like a hot knife through butter. As the set name implies, this set best works in the rain. Waterfall is Buizel's main means of attacking, with decent Base Power, STAB, and perfect accuracy. Aqua Jet allows Buizel to also function as a revenge killer outside of rain, picking off faster, weakened threats before they can attack. Return pairs well with the previous moves, as the combination of Water and Normal is unresisted in this metagame. Finally, what goes into the last moveslot boils down to what you want Buizel to hit harder. Bulk Up is the primary option, as Buizel appreciates the extra boost in power, but Ice Punch is a decent substitute because it hits Dragon-types such as Dratini and Bagon harder in the rain, even OHKOing the former. Outside of rain, Ice Punch hits Gligar harder than Waterfall, though it will only OHKO the defensive variants 50% of the time. Finally, Brick Break guarantees an OHKO on Carvanha, and outside the rain, does more damage to Pokemon weak to Fighting, such as Aron and Munchlax. A Life Orb is essential on this set, giving Buizel the extra power that it needs to sweep efficiently. The EVs maximize Buizel's Attack and Speed, and the remainder is used to give Buizel an extra point in Special Defense.
This set loves Rain Dance support, because Buizel needs the extra power on its Water-type attacks the Speed increase for sweeping. Six turns of sweeping is generally enough for Buizel to tear holes in the opponent's team. Great Rain Dance supporters include, but are definitely not limited to: Voltorb, Bronzor, and Drifloon. This set also greatly appreciates Stealth Rock support, as it can turn numerous 2HKOs into OHKOs. For example, Buizel can OHKO Carvanha with Return if Stealth Rock is up, so it no longer has to rely on Brick Break for the guaranteed KO, though it should be noted that Buizel would be switching into Carvanha, not the opposite. In addition, with Stealth Rock on the field, Buizel 2HKOes Swift Swim Mantyke with the combination of Aqua Jet and Return.
Though this set has great offensive potential, it still has flaws and solid counters. For one, Snover simultaneously resists Water-type attacks and puts an end to Buizel's rain. As long as Snover does not switch into Brick Break, it can OHKO Buizel with STAB Energy Ball. Mantyke can revenge kill this set with Hidden Power Electric. Mixed Priority Abuser Croagunk absorbs all Water-type attacks aimed at it, and can KO Buizel with Fake Out and Sucker Punch, but only guarantees itself an OHKO if Buizel has switched into Stealth Rock and taken at least one turn of Life Orb recoil damage. Munchlax can take on Snover any day of the week, and can easily revenge kill Mantyke. Bronzor is a great teammate for Buizel if Croagunk is a problem - it can take any attack that Croagunk can dish out and OHKO back with Psychic.