It depends on what other characters you have up and running, ready to make a team. Do you have other Rogue One units ready? Then Raddus. Do you have a Kyle, Pao, HRScout, or other Rebel Fighters ready? Then MM. Do you have Wedge, Biggs, Chirrut, and Baze (and Luthen?) ready? Then Saw.
But maybe the better question is who fits the direction you're heading. Do you need Jedi Master Luke? Then Mon Mothma. Are you working toward the Profundity? Then Raddus. Saw isn't a requirement anywhere.
And in answering who fits your direction, the best answer may be none of those. Instead of looking at just trying to 7 star everyone, look at what your next journey or other goal is. Who on your roster, even if they're already at 7 stars, may need that LST to get you closer to that goal?
Or depending on how new you are, you may just want to use the LST to strengthen or fill a gap on one of your core teams.
Or you could always just make a passion pick. Do you really love one of those characters or their source show? Then consider that one. At the end of the day, you want to have fun and enjoy your teams.