Nobody mentioned the standard Fire/Water/Grass core?
Still seems pretty effective to me imo
I used to run, uh... Lemme find it.
I'm not sure if this is classed as offensive, though... It looks like it should be. Worked like a charm in a majority of the battles I used it in. The theme I generally have in all my teams is covering weaknesses, so, that's the idea here as well. Two steel-types were also on the team as well as Latias for the Levitate and multitude of resistances. The entire team was a "core", so to speak, but this was what I started with. A different version of the team exists though, where Milotic was replaced with Suicune (or maybe it was the other way around).
More offensive, although still did the same job of being a physically bulky water-type and definitely helped with the resistances/immunites theme I always have.
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Also, one defensive core I (potentially; that is, I didn't look anywhere for it and believe it to be my own creation...sorta) came up with myself:
Self-explanatory, I think, given the natures and EVs... Although Cresselia is meant to set up more than just be a wall, it does take Physical hits extremely well; just as Porygon2 takes Special hits the same way. They also cover each others' weaknesses, sans Dark, but outside of Sucker Punch, I think Ghost is chosen over Dark where possible (or logical) in terms of offense anyway, isn't it?
It's no SkarmBliss, and Moonlight isn't a very reliable recovery option, but it's something.
...and I just really love these two Pokemon.