The Doctor has had many loves; some have been brief while others had spanned years and galaxies. But there are three that hold a very special place in his hearts. They all are "true love".
Disclaimer: I do not own Doctor Who or any of it affiliated characters.
The First
His wife on his home planet Gallifrey will always hold a special place that cannot be replaced. She was the first one that made his hearts pause when she smiled. She was the one that had first given him an understanding of all the beauty that the universe could hold. With eyes closed, he can still see her face as she gave him the gift of being a father. This of course he never did since the pain is far too great, to see what he has lost. She was his First.
The Rose
Rose, such as her name describes, was beautiful and grew more beautiful as she blossomed. The hard hearts that beat in the his chest were slowly made tender as Rose rooted herself in them. After the war, after what he had done, beauty had become only a concept, something he strived to truly see but could not achieve. With Rose, his only goal with her was to show her the beauty of the universe and ride on her emotions as she saw them with innocence. His purpose was for Rose to be a catalyst to help him at least experience awe and wonderment as a spectator. He had never planned for Rose to do so much more. She was the reason for smiles to return, feelings of slight jealousy, desires to be viewed as the best and to be a protector. At the end the his hearts had awaken enough to care for Rose, but also enough for him to love her enough let her go. She was his Rose.
The River
Flowing unpredictable was the woman who lived up to her name, River. This was a woman who brought with her a flood of emotions that confused him from the beginning of his meeting her. The first meetings were met with suspicion, followed by curiosity and continued to move to flirtation. River didn't seek to see the universe for only its beauty but for its adventure. She held her own, didn't need a protector, but would protect at all cost. She kept him on his toes, played along on his level, intrigued him, and challenged him. River became the second woman to hold the title of wife, and the first in several hundred years to know his name. River flowed through the his hearts, etching a permanent place and seeping into places he never expected. Despite his knowledge of her end, with each nightly adventure, with each kiss, with each flirtatious banter, his love for River developed into a mature love, a love whose stability contrasted their time streams. She was his River.