>>183735304. Ability. Having something useless like Pressure won't drag you down, but it can't help you survive. Defeatist, Truant, Slow Start means your shot of winning is even more narrow than most other pokemon. But having an ability that gives you an immunity means that's one less entire type that can be used against you. Unless they've got mold breaker, which is also quite handy. Some might be helpful in different ways; Natural Cure would activate after each fight (I think), Immunity would save you from being poisoned, Regenerator would ease your recovery, Tinted Lens would let you use less attacking moves.
5. Stats. You must have some really good stats. Even with a 4x weakness to Fighting for example, Tyranitar remains a heavyweight because it can actually survive some non-stab fighting attacks and retaliate on either the physical or special side. Being specialized isn't such a good idea either; having great defense but terrible special defense would leave you wide open; a Steelix could shrug off most any physical attack, but any special attacker could make mincemeat of it. This is true on the offensive side as well; a pure, powerhouse physical attacker might get stuck against Skarmory, or a special attacker unable to get past a Jellicent. A mixed attacker pokemon could wallbreak much, much easier.
6. Item. Being into an item that locked you in would just hinder you. The choice items would be obviously not chosen. A life orb without recovery would be suicide after you make ten attacks, not including any damage you take. An Expert Belt or Muscle Band / Wise Glasses would aid an attacker, Leftovers would be universally useful. One use items such as Balloon or Focus Sash would suck (unless you have recycle or maybe Pick Up), but maybe a Focus Band would save you just when you'd need it. Or not. A quick claw could help you make a killing blow fast enough to avoid taking a dying pokemon's last hit, or rapidly status an attacker before they strike.
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