Sunless Citadel: start level 1; advance to 3
Forge of Fury: start level 3; advance to 5
Lost Shrine: start level 5; advance to 7 or 8
White Plume Mountain: start level 8 advance to 9th or higher
Dead in Thay: "designed for characters of 9th to 11th level"
Against the Giants: "designed to be undertaken by characters of 11th level." - also, says should expect to level up after each section, so 14th level after finishing the fire giants
Tomb of Horrors: "Only high-level characters stand a chance of coming back alive" - having read it I don't think a party of less than 15th level has a chance, and even those parties will likely suffer one or more character deaths. I'm planning to seed rumors about this place in our current campaign (characters range in level from 5th to 10th) and let the PCs go whenever they think they are ready.