i belive marneus calgar is metal but i have herd whispers of marneus calger being avalible in plastic. apparently REALLY BAD FEATURES.

Either, Angraft, Because it's big, or the Thunderhawk, because it's big, or some famous Space Marine captain like Marneus Calgar or Darnath Lysander.

It depends on what you want. Marneus Calgar might be the one with the best abilities, along with an impressive statline. But he's too expensive. It really depends on the type of abilities you have.

If your talking fluff wise then probably the Emperor, but in game, it depends on who you ask due mainly to the fact that everyone thinks their things are the best. One of them could be Marneus Calgar with 2 full squads of honor guard.