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Assuming OP's talking about Smogon OU:
Have at least one check for every S and A+ rank OU pokemon (that is, being able to beat them in a straight-up fight without team support).
If you're going full offensive, having synergy in your mons' resistances to switch in on various predicted attacks is certainly nice, but not something you should rely on too much since your team's going to be pretty frail anyway.
Having Stealth Rock on your team is absolutely mandatory. Setting it up during every battle is not, but if your opponent goes for setting up his own hazards and play a switching game to wear you down, you're going to be screwed if you can't do the same to him. The only exception I could think of is when you have a magic bouncer that can potentially send your opponents' hazards back. Spikes/Tspikes stacking is an entirely different strategy that does not work if a large chunk of opponent's team consists of fliers and/or levitators, so don't make a Pokemon fully specialized in setting them up or it's going to be a dead weight in a large chunk of your games.
Hazard control (spinners or defoggers) is mandatory if one of your mons is weak to pebbles, but just a bonus if your team takes neutral or decreased damage from them.
