It depends on how you define 'toughness' because if you are talking about capability wise then they still don't get much tougher than Samus Aran for her giant laser canon blasting greatness.
But if you are talking about tough as in personality, that gets a little bit more tricky, as toughness in personality is somewhat harder to define:
But still, my answer to this is the same as it has been in every 'favorite female character' and 'tough girls in games' question for the last few months. Ashley Williams of Mass Effect.
This chick has it all, tough no-nonsense attitude, determination, anger and passion and a real human personality that is both reinforced and occasionally poisoned by her aggressive stance.
Not portrayed as a sex symbol, nor does she ever turn into a blob of jelly even when discussing more personal emotional issues of hers and most of all is very capable in combat, being second only to Wrex (the KROGAN BATTLEMASTER) in terms of weapon usage and ability to use them well and while you may never see her leap atop a mech in slow motion dual wielding pistols, you will see her determinedly standing her ground and fighting it off.
Seriously, if you ask me, despite the hate she gets, I reckon Ashley is a brilliant example of a strong yet humanly flawed female character in games and a great example of toughness.
As for female characters in fighting games, well, Samus was in Smash Brothers so I'll count her.
