I sucessfully use the following Quest addons on all

Doublewide:
- name says it

Questhelper:
- TomTom navigation
- Quest items

Cabrbonite (non commercial): (I am not going to pay $ 2 a month for an addon !)
- for the map and the area

having carb on the alts is also nice to see what they where they have to loot, if you accidentially loot all on the main already.