What are some good defensive Pokemon combos?
Do you want defensive cores? IMO the best in the game today is Skarmory+Heatran+Gastrodon+Latias
Some other great defensive cores:
Gliscor+Jirachi+Rotom-W
Ferrothorn+Gliscor+Jellicent
Skarmory+Latias+Tyranitar
The worst defensive core in 5th Gen:
Skarmory+Blissey, Blissey literally contributes nothing, and gets beaten by pretty much every special attacker anyways now because of Psyshock, Trick, Lum Berry, Taunt etc. Never use this combination to create a defensive core, the only time you should see Skarmory and Blissey on the same team is if that team wants Spikes and Wish support.