This is one of those questions with no defined answer - some people might think that those with high HP but not as great Defense stats, like Wobbuffet, are the more resilient, but when you come to stuff like Dusknoir and Deoxys-D which have low HP but high Defense stats, those might appear more resilient. But now that we have Mega Evolutions that can really throw a punch, with the low HP it might not feel like much Defense. But some might like the greater Defense to their advantage even without the HP to back it up. Although back in Generation III, Blissey was one of the biggest walls ever given its massive HP and Sp Def, despite its paper-thin Defense (you still needed a very strong physical attacker to handle it properly).