Okay but Donna was one of the best companions in my opinion. She was strong and independent. She had human compassion and was so caring and loving, but at the same time she was strong and she could push through difficult things. More than that, she pushed the doctor through difficult things. She challenged his convictions but also helped him to make hard and painful decisions.
She was there at Pompei, and helped him trigger the eruption of Vesuvius, so that the burden wasn’t his alone. But she also managed to convince him to save just one family from the tragedy. Even when tempted with her own future, she discouraged him from reading River’s diary in the Library. She spares his “special Timelord code” of “not very alright at all” when she was grieving the loss of her family inside CAL’s simulation.
For fuck’s sake, she becomes a Timelord, and keeps up with him. And the absolute best thing about her was that she was exactly what the Doctor asked for; she was his mate. She was a friend and not a love interest or a fling, she was just a mate.