Just a quick note here: FW did not "create" the Carcharadons. The Space Sharks have been around since the (original) Rogue Trader era.
Ultramarine are my favorite. Have been from the beginning and I've remained loyal to them ever since.
Full-length stories about Space Marines do them no justice; once you get to know the character and see the character in action, everything else is a broken record. Short stories do a better job of relating the uniqueness of any particular Marine.
Least favorite is the Rainbow Marines. Sadly, originally, they "spawned" the Ultramarines.