Depends on the creature you're fighting, their Resists, and whether they're vulnerable to a Slayer or not. Remember, Poison Strike, Wither, Mind Blast, and Earthquake are not affected by Slayers, and MB and EQ aren't affected by SDI either, so even if an opponent might be weakest to that element, another spell might do more damage due to Slayer spellbook/SDI.
For single target Physical Damage, Mysticism's Bombard is affected by both Slayer and SDI, so is a clear winner there. The only other Physical damage spell, Earthquake, has it's damage scale with the victim's current HP, the lower their HP, the less damage EQ deals.
For single target Fire damage rotation, Explosion+Flamestrike is the winner (Exp+FS kills faster than EO+FS, and kills at about the same speed as straight FS spam, for less Mana cost).
For Cold damage, Hailstorm is affected by Slayer and SDI, but it also splits damage between multiple targets. Harm is also affected by Slayer+SDI, but requires point blank range to deal it's maximum damage, which is risky. Wither isn't affected by Slayer, but is affected by SDI, it deals full damage to all targets hit.
For Poison Damage, there's not a real good option in most circumstances, even with Poison Strike. Poison Strike isn't affected by Slayer, but is by SDI. Against opponents with Poison Resist far lower than their other resists and without a Slayer, then Poison Strike can be viable. Travesty is one of the few instances where Poison Strike actually shines, especially in combination with Conduit. The splash damage of Poison Strike will kill the Mirror Images while also damaging Travesty herself, and Conduit+Poison Strike creates a cluster bomb effect where Travesty is getting slammed with multiple Poison Strike splashes.
For Energy Damage, Death Ray has some restrictions on it's effective use, but it deals heavy Energy Damage and is affected by Slayer Spellbooks (although not SDI). Since Death Ray also lowers the victim's Energy Resist by 10 (even into the negatives), it boosts everyone's Energy damage against that victim as well. E-Bolt is also a good option for when Death Ray use isn't feasible due to movement/AoE damage or when using another Mastery. A pet's "Conductive Blast" ability halves the victim's Energy Resist, which makes Energy Damage very effective against virtually any foe, especially when paired with Death Ray's Energy Resist debuff.