Destruction Warlock can spend 5 points in 0/13/58 spec to bring Replenishment (via Improved Soul Leech) to the raid.
Replenishment gives 10 party or raid members 0.25% of their maximum mana every second for 15 seconds. It is recommended to have minimum one replenishment class for Ulduar 10 and two for Ulduar 25. The buff is especially helpful to healers.
Other choices are
Shadow Priest - Vampiric Touch
Retribution Paladin - Judgements of the Wise
Survival Hunter - Hunting Party
Frost Mage - Enduring Winter