There are four buff/debuff sets that have multiple -res powers. Poison offers the highest total vs. single targets and Cold offers the highest total vs. multiple targets, although Heat Loss has a 6 minute recharge so can't be made perma. There are two blast sets with -res. Sonic offers the most, as 6 of its 9 attacks provide -res, but Beam also provides -regen. The highest -res combo vs. single targets would be Poison/Sonic. These are all defender numbers:
Poison
Envenom: -40% (ST) / -20% (8 ft AoE)
Venomous Gas: -25% (15 ft PBAoE toggle)
Sonic Resonance
Sonic Siphon: -30% (ST)
Disruption Field: -30% (15 ft Ally AoE toggle)
Cold
Sleet: -30% (20 ft AoE)
Heat Loss: -30% (20 ft AoE)
Trick Arrow
Entangling Arrow: -20% (ST)
Disruption Arrow: -40% (25 ft AoE)
Sonic Blast
4 ST, 1 cone, 1 AoE: -20% each (stack with each other), 5-12 sec duration (20 sec for Dreadful Wail)
Beam Rifle
Piercing Beam: -20% (5 deg cone), 10 sec duration