Last seen companion ever is C2-N2 / 2V-R8 + HK-51 (Probably becose not everyone has him at ALL chars)
P.S.: When i was lvling my marauder i picked Broomark and since then i never left ship without him, i NEVER used Quin and i never had problem with any mission/soling H2. (Its maybe cose i like to see my loyal polar bear in full rakata mixed with BH runing around )
When i lvled my JG i was using HK-51 all time (dps spec JG + HK-51 = fights ended before they started)