but in case you're serious the best defense for women or other weak persons are these types of martial arts which uses force of the attacker against him. Aikido or Tai chi would be probably the best for you. I'd suggest also to think about stick (long or short) fighting classes - very effective even against the knife.
Hard styles like karate, kung fu are really not for women IMHO unless you want to have lot's of bruises, hardened hands, feet etc...
Remember - the best style is fighting without the fighting.