I searched the forum and I did not see any threads based on this. What are your great defensive cores? Let's go beyond the common realm of SkarmBliss and FerroCent. I have recently discovered a great core for you hax lovers with great synergy.

Dragonite @ Leftovers
Trait: Multiscale
EVs: 252 SDef / 252 HP / 4 Def
Careful Nature
- Substitute
- Roost
- Dragon Tail
- Thunder Wave

Jirachi @ Leftovers
Trait: Serene Grace
EVs: 80 HP / 252 Atk / 176 Spd
Jolly Nature
- Substitute
- Body Slam
- Iron Head
- Fire Punch

Dragonite paralyzes everything besides ground types, which Jirachi paralyzes with Body Slam. The only immune Pokemon are Golurk, Golett, and Limber mons. Nothing too common. Dragonite's typing gives it weaknesses to Dragon, Rock, and Ice, all of which are resisted by Jirachi. Jirachi has only 2 weaknesses to Ground and Fire, one of which Dragonite is immune and the other it resists. I based a team off of this and it's a fun team. I'm making an RMT soon for this. Back on topic, name your defensive core of choice.