The most universally shareable heirlooms are going to be the cloth chest, cloth shoulder, and ring items.
This will provide a 10% XP bonus when equipped and can be used by every single class. While the stats may not be the most useful for all classes, the 10% XP benefit will greatly outweigh the small stat loss you would have for not having gear catered to your spec in these two slots (chest and shoulders):
Tattered Dreadmist Robe
This will provide a 10% XP bonus when equipped and can be used by every single class as well. I would recommend the PvP shoulders because they have both crit and resilience, which will be more useful for all classes than spirit and intellect will be.
Exquisite Sunderseer Mantle
However, there is also the PvE chest that would work just as well in terms of providing the XP boost for all 10 classes.
Tattered Dreadmist Mantle
Although the stat bonuses they confer are not going to be useful for all classes, if you want to just buy one set for any character, these would work best.
Finally, if you can manage to win the Kalu'ak fishing derby, you can get an extra 5% XP bonus. This item is great for all 10 classes as well.
Dread Pirate Ring
The Haste Trinket will work well for all classes, as they can all benefit from haste in some way. However, for the six classes that can level via a caster class, the Spellpower trinket will work better (Mage, Warlock, Priest, Druid, Paladin, Shaman) and you can use two of them at the same time.
The only other slot that has available heirlooms currently is the weapon slot. This slot will be much harder to fill across all the classes. You will more than likely want to save your emblems for weapons last, and buy them for whatever class you plan to level (and in some cases, which spec).