I'd probably chose Liara from Mass Effect 1. There are multiple reasons for this:
A) She's capable of living for at least a 1000 years, so she'll be around a long time. (OP doesn't specify whether the companions are immortal)
B)Recreational activities
C)She would carry with her at least two weapons of the Mass effect (1) kind, so basically, infinite firing capabilities and very deadly.
D)She has biotics which can be used to very effectively protect us for a very long time, and she can also use them to make the human kind worship her as a deity, hopefully me as well.
A) She's capable of living for at least a 1000 years, so she'll be around a long time. (OP doesn't specify whether the companions are immortal)
B)Recreational activities
C)She would carry with her at least two weapons of the Mass effect (1) kind, so basically, infinite firing capabilities and very deadly.
D)She has biotics which can be used to very effectively protect us for a very long time, and she can also use them to make the human kind worship her as a deity, hopefully me as well.