The best defensive character in neutral has to be Tempest Kung Lao , jump back 2 and EX spin not only cover anti-airs but the rewards is always over 30% into knockdown pressure , f2 and d4 ( and j2 ) regulate the ground approach and orbiting hat literally put a shield in front of you. Second pick would be Grandmaster Sub , literally denying ground and air approach with EX clone , good d4 and answers to frametrap and airs attacks with EX ice shatter wich is mostly safe due to the awkward blockstun on it , slide does negate quite a few projectiles too along with iceball for trades.
Best defense from frametraps has a lot of contenders , Cassie w EX flip , s2 , d3/d4 , Blood God w obsidian totem for meterbuilding and extended lifebar + jump 1 and parry for projectiles and gaps in strings , Kano with b1 for ground and air control , EX upball for minimal risk ( basically can't be armor break that ) on frametraps and anti airs , great d4 in neutral too , Cristalline Tremor with + on block launcher for AA or ground approach , f1 7 frame high and rock shower in neutral , Aftershock loses the Shatter but gains great anti zoning.
There isn't really a definitive answer , best defensive options do change based on the matchup you're playing , for example i'd rather defend with Kung Lao strong neutral game than Cassies answers to frametraps / pressure when fighting a character like Quan Chi just because when he gets started Cassies options don't matter.