Dead Money companions. Period. No seriously, their backgrounds and stories are amazing. Sure, they are not regular full-time amigos, but I think they're so well explored, especially if you look on game as a whole (meaning vanilla + DLCs). That's not to say that companions from vanilla or other DLCs are bad (okay, Honest Hearts could've gone better on that one), but even though I spent much more time with Boone, Cass etc., they didn't remain in such a memory as the aforementioned ones did.