The trick to a good boosting instance is being able to leave your alts in a safe spot while you clear with your main. This allows you to focus solely on clearing it as fast as possible without worrying about if they die etc. Also, a dungeon that you can clear in a circular fashion is better to minimize run time. To that end, here's my boosting path:

1-10: Questing.
10-20: SFK. You can leave your alts at the zone in portal and clear the whole instance. On your way back you can save about 2 minutes by jumping onto the roof and then hopping down into the zone in area.
20-35: SM. With my DK I can clear all three wings in less than 12 minutes. You can't leave your guys at the entrance in any of the wings and still get XP but you only have to move them about halfway through on each one--I clear it to half and then move them up while I gather the rest of the mobs. Cake walk.
35-45: ZF. The slowest part. You can clear straight up until you get to the T junction and leave your guys there. Go clear the rest of the instance (it's a circle) including all of the tombs with the zombies in it. Then you can mount up and reset.
45-55: LBRS. I don't know why people take Strath or Schol over LBRS. You can zone in and clear the entrance room and then go down to the drop off and leave your alts there. Then you can drop down and clear all of the orcs and then move your way up through the troll area all the way to your alts. Alts get XP the whole way and you only spend about ~45 seconds inside the dungeon where you're NOT killing mobs.
55-60: Ramparts. I tend to bring alts with me but you can move them forward across the bridge and they'll get XP for the whole instance.

My fastest boost was just over 7 hours. I think you could cut an hour off of that by not wiping and somehow speeding up the 1-10 portion.