Well, Mira and the Handmaiden are mere padawans while Visas was a Sith apprentice with at least a few years training. And while both the Handmaiden and Mira have a strong family force connection, Visas' entire race manipulates the force as easily as they breath. Now, let's look at it like this:
Mira
+ Bounty Hunter and a damn good one. Good at stalking and scrapping. Not adverse to being underhanded.
- Relatively new to the Force, and no exactly having the control and wisdom to be a real danger as a Jedi.
Handmaiden
+ Echani training in unarmed and armed melee. Definate plus there.
- Inexperience with people and general open hoenst approach to life makes her susceptible to trickery.
Visas
+ The most Force experience here, as well as an exceptional command of it. The only one trained in Sith ways.
- Less physically imposing. Not exactly a fighter at heart, and has perhaps less lifelong training in combat than the other two.
Visas knew the force all her life, but there is nothing that would suggest she had been using it for long. Or that she was picked as Nihilus his apprentice for anything more then her sight...
I say Handmaiden takes this, she has most training and experience. Maybe not as much control of the force as Visas or Mira, but neither of them are really spectaculair either.
To see yes, but she had not learned any other technique she was not a powerful force user. She could not shoot lighting at others, she could not use the force for her. The force for her was like glasses are for many people here. A tool to see nothing more, nothing less.
She did not learn to control the force until after her planet blew up. At least she did not know much before, i will not say that she wasn't powerful. But the only reason she was Nihilus his apprentice was so that she could see for him. Otherwise Nihilus would have just killed her. She was not extremly skilled she can be quite powerful sure, but she is no master. Handmaiden simply has a better fighting style, and will win because of it.