If we're talking about combat sports overall, she definitley is...
She is the first ever person in history to win major World Titles in boxing and in MMA...not just woman, but person period. She was Ring Magazine Fighter of the Year twice and she is undoubtley one of the greatest female welterweights of all time in boxing.
She knocked out the first ever women's champion in UFC history....She almost became the first two division UFC champion in Women's MMA history.
Lucia Rijker is definitley up there, but being a World Champion in boxing/kickboxing is nowhere near as impressive as being World Champion in a striking sport as well as a sport that involves grappling like MMA.
Holly is easily one of the greatest female fighters of all time, no question.