There are characters we fall in love with. The perfect mix of actor and writer that breathes magic onto the screen or inside the television.
Martha Jones was better than good. She was the best. Dr. Martha Jones met The Doctor when her hospital was transported to the moon by the Judoon in search of a missing prisoner. Not only did she save The Doctor's life, but Martha Jones would go on to save the Earth and defeat the Master without The Doctor's help. Think Donna Noble or Amy Pond could have accomplished that?
Unlike The Doctor's other companions since Doctor Who's relaunch in 2005, Martha brought an unique skill set to the party. She wasn't a temp or shop girl. She was a fully capable medical student who was made stronger by her travels with the Doctor.
Martha Jones also stands out among all the companions is that she wasn't "special." Martha didn't grow up with a crack in time that changed her reality. Nor was she destined have a meta-crisis or be Bad Wolf. Martha was just Martha, and in the new Doctor Who that sets her apart completely (with the possible exception of Mickey).
Some fans disliked Martha's mooning of The Doctor and her unrequited love that defined their relationship. Those feelings were only finally put to rest by living out the year that never existed and realizing that she was strong enough to live without him (something, I'll point out, Rose Tyler never accomplishes). Of all the companions Martha's life is enhanced the most while still being able to leave on her own (without suffering the negative effects felt by Rose, Amy, and Donna).
After Martha left The Doctor she carved out a new position for herself in UNIT and returning to the show for a pair of appearances to help The Doctor stop a Sontarian invasion and fight off an armada of Daleks. She also turned up on Torchwood and, at some point, went into the freelance alien fighting business with her new husband Mickey Smith.
Played brilliantly by the beautiful Freema Agyeman, Martha Jones is one of the most under-appreciated characters in the Doctor Who universe. Smart, capable, and (like all of the best Who companions) a woman who listened to both her head and her heart, Martha could make the hard decisions (even if that meant sacrificing her heart's desire). For those reasons, and so many more, Martha Jones is awesome.