Doctor Who: Every NuWho Companion Ranked From Worst To Best
2. River Song
The saga of River Song is one of the better additions that Steven Moffat brought to the show both before and during his tenure as showrunner. Alex Kingston is pitch perfect as the enigmatic Dr. Song, first appearing in her (almost) final day with The Doctor, before popping up again and again through time.
River is able to speak to The Doctor like no other, showing the love and respect that she has for him, but understanding him in a way that no one else does either. Whether she is gifting him all of her regenerations (just go with it) or trying to murder him, River Song always makes life interesting for whichever incarnation of The Doctor she's dealing with at that point.
While her story does feel complete at this point, never say never when a TARDIS is involved.