My last, and fourth, 6* was my Shannon (Wind Pixie) and that allowed me to reliably complete Giants B10. She is kinda squishy as a 5* and the ~2k base life she gains from 6* makes it worth it. I don't even have her runed despair and I'd still say it is definitely worth it (Mine is swift/focus). Although she is extremely useful, I would never recommend her being the first 6* anyone makes.
I'm going to echo Aevry and say that I would not 6* any of those monsters in the short term. Most people don't take Megan (Water Mystic Witch) past 5* that I've seen and Ramagos is great early as a 5* and can be amazing as a 6* with godly runes, but dies off in usefulness in the mid game. I haven't leveled mine past 5* yet.
Unless you get a good pull, I'd really look into farming Faimon until you pull a Roaq (Fire Inugami). He is a solid farmer and a solid first choice for 6* if you don't manage to pull a better farmer from scrolls. You need someone that will be able to efficiently farm one of the "main" exp farm spots: Hydeni 5, Tamor 4 or Faimon 1. Ramagos can probably do Hydeni but will be extremely slow.
I do recommend taking Lu Lu (Water Howl) to 5* in the short term. His healing is pretty amazing early on but dies out a bit later, and eventually the utility that Konamiya (Water Garuda) brings is more valuable. I do think that some people use him in upper TOA, but I currently haven't gotten past floor 53.
BTW, feel free to send me a friend invite (IGN: Rothir). My Acasis (Wind Sylphid) can pretty easily solo Hydeni 5 Hell.