I have the undoubtedly unpopular vote for
Son of Drogo. He's not in nearly as many of my decks as
Courteous Halfling,
Old Bilbo's Heir, or
Reluctant Adventurer, but he's one of few Frodos where choosing him matters. Signets aside, most of the time those guys are just a Frodo without much contribution: I can sub out
Courteous Halfling for
Old Noakes, or
Old Bilbo's Heir for some other protection, or just let a extra 3 tokens wind up in the twilight pool over the course of the game. Sure they tilt your deck in a certain way, but for the most part decks with them could function almost as well without them. Because they're so generally useful I understand why they're going to be a lot of peoples' favorites though.
Sam, Resolute Halfling;
Merry, Learned Guide; and
Sting, Baggins Heirloom + Smeagol (especially if you're not using
Slinker) are the most obvious combinations with
Son of Drogo, and each are a lot of fun. Instead of
Pippin, Hobbit of Some Intelligence removing one wound and then dying,
Friend to Frodo can remove three. The Morgul Orcs or Twilight Nazgul are pretty much doomed against him, and if you have healing for other cultures (looking at you, Elves) now you can have healing for Frodo too. I consider the signet interaction as a nice bonus rather than a handicap, makes choosing your main fighters a little more interesting and gives some life to one of the most under-utilized aspects of the game pre-Shadows.