We have a
Viability Ranking Thread with rankings of Pokemon + write-ups to show what stands out among the rest and why.
Now let's talk about cores. There are many interesting cores; you can be very creative with both offensive and defensive synergy.
Rule number one is to be creative with cores. For example, I tried a Lightningrod Rhyperior + Choice Band Gyarados in Sandstorm Trick Room the other day because I am stupid, and it actually worked out kinda ok-ish considering how shitty it sounds. Great standout cores are as follows
The following are cores with a supportive mon and a sweeper(ish)
Heatran + Cresselia. Sunny Day and Icy Wind from Cress help Heatran so much.
Hitmontop + QD Volcarona. Wide Guard + Fake Out is honestly all Volc needs to sweep non-Rain teams.
Togekiss + Garchomp. Follow Me support, Tailwind / T-Wave support, potentially Helping Hand support, and immunity to EQ. Garchomp might as well propose to Togekiss.
The following cores have cool defensive / offensive synergy.
Mamoswine + Zapdos. A classic "DisQuake" core. Basically, you spam Earthquake and Discharge and hope to god that the opposition doesn't have Wide Guard. Zapdos resists Fighting and Grass, which Mamo is weak to. Mamoswine resists Ice, which Zapdos is weak to. Offensively, Ground + Electric is only resisted by some Grass- or Ground-types, which get popped by Ice-type moves from Mamo.
Hitmontop + Thundurus. Say hello to your (or your opponents if you are using this core) living hell. Though they may not have the greatest typing synergy, the way they support each other at cutting off almost all opposing strategies is beautiful. Fake Out + Thunder Wave / Taunt on turn one makes it easy to cripple a couple of Pokemon; get a headstart you could say. From then on it's all about spamming Gem CC against paralyzed opponents (top is slow as balls), and paralyze the Pokemon that are not paralysed. Elegant goodstuffs core.
Fuck it I don't feel like thinking of more, but there are plenty. The point is, it's quite easy to find mons that work well together, and most Pokemon will work well together unless running a spread move that hits yourself (example: Follow Me Jirachi + Earthquake Excadrill. Or if you are running monotype lol.
Hope I helped a bit :)