From personal experience, I would say a Prot Paladin is best for boosting from 10-45, a Sweeping Strikes Warrior from 45-65-ish, and quests from 65-70. That somewhat depends on what you're boosting. Up to 45 a prot pally pulling half an instance or a full instance and Consecrating them down will be the fastest method. After that, the Warrior will take over as pulls can't get to large, and their DPS will be far superior to small groups. Of course, you need some healers to keep the Warrior going, so if for some reason you weren't boosting a healer, you CAN stick with the Pally to 55, but its harder.