A team that only focuses on countering pokemon will have limited success, because in order to win games you must not only be difficult to destroy but also difficult to counter. However any team, first and foremost, needs to be able to reasonably survive against opposing teams. For this reason a defensive core is often the basis for a team. The rest of the attention will be focused on rounding out the team's counters while creating threat potential in the team.

Discuss, create, and critique defensive cores.

SAR Core
Scizor / Vaporeon / Rotom-A
CB Scizor ~ Wish Vaporeon ~ Standard Rotom-A
Scizor and Rotom-A cover a plethora of threats. Together, however, they don't really have any special kind of defensive synergy. Vaporeon is the glue here. It rounds out the defenses with standard bulky water play while offering wish support which both Scizor and Rotom-A appreciate a lot. It also helps counter Infernape who loves to rip through average defenses. Rotom-C offers the greatest type coverage to handle bulky waters, but thunderbolt is often enough. Rotom-W is best against Tyranitar but redundant in type coverage. Rotom-H takes care of steels better. Many things are left unchecked and will have to be left up to the rest of the team to prepare for. Jolteon comes to mind.
Consider: Would Heatran be better in the place of Rotom-A?

Obi Stall Core
Hippowdon / Skarmory / Tentacruel / Blissey / Celebi / Spiritomb
Blissey and Skarmory cover a huge number of threats. Infernape, which counters both of them, is covered by Tentacruel. Celebi takes care of things that want to set up indefinitely. Spiritomb counters rapid spinners. Hippowdon is a secondary physical waller. Lucario isn't directly countered but can be eliminated through residual damage and smart switching.

Also offense can be a form of defense. If you have a pokemon with few counters, then you only need to prepare to counter a few pokemon when you have that pokemon out. Such is the idea behind heavy offense teams. I created a team based on a Kingdra/Empoleon/Lucario core that works in this way.

I'd use rotom-h over spiritomb as a spinblocker in OU...