Gen V only
248 HP, 8 Defense, 252 Sp. Defense
- Avalanche/Ice Shard
Don't use this set in gen VI, as it won't get far. This is because it requires permanent rain to work(Unless you were to keep up Primal Kyogre in a double or triple battle, for some reason). However, in an appropriate environment, Hydration becomes an immensely powerful defensive Ability. Not only does it give an essential immunity to poison, and a near-perfect immunity to Sleep, Burn, and Paralysis, it also makes Lapras go from having no recovery moves, to having a powerful, 100% complete restoring move in the form of Rest. Restoring 100% of your HP as apposed to just 50% cannot be underestimated, as it means anything that doesn't 2HKO it not only can't stop it, but can't even prevent it from doing whatever it wants; it will just alternate from X move, then Rest and bring itself back to full HP. Hazards and prior damage are rarely significant as well. With Special Bulk nearly equivalent to that of a Specially Defensive Sylveon or Umbreon, not 2HKOing it from said front without a Super Effective STAB is an absolute possibility. The combination of HydraRest alone makes Lapras very difficult to stall out due to its immunity to status and very hard to kill outright, but it gets worse. Curse allows Lapras to bring up its physical Defense to similar levels to its Sp. Defense and beyond, letting it take physical hits well and also boost its Attack, making productive use of being able to do "Whatever it wants". Its most notable weaknesses are to Fighting and Rock, but once it cushions those moves to manageable levels your opponent will be in serious trouble due to Hydration. Waterfall is STAB and takes advantage of Curse's Attack Boosts, as well the fact that it gets boosted by rain to give it an impressive 120 BP without drawbacks. Lapras may only have 206 Attack, but it has a much better 309 Attack at +1 and a relatively impressive 412 Attack at +2, meaning after a boost Waterfall with definitely hurt. Avalanche has secondary STAB, and also has 120BP if the opponent Attacks Lapras, and overall Water/Ice gets good coverage. Ice Shard can be used to quickly nail Grass Types that might try to hurt Lapras, and to some extent Electric Types that are weak to Ice, especially after it has gotten a boost.
This set might be outdated to some, but at the same time it single-handedly made climbing the Gen V Smogon OU ladder extremely easy as if it were some sort of cake walk, and not just me, but pretty much every friend of mine who knew about it. Rotom-W and Ferrothorn might look good against it on paper but both of them generally can't handle Lapras once it comes down to it, unless they use relatively abnormal and unusual, non-standard sets. So honestly, I had to do this at some point.