I've shared my opinion on a couple of Marvel characters who could beat Cass. Cass defeated? Yes, it has to happen. Otherwise she'd just be invincible and never in any kind of jeopardy, which renders a lot of the external conflict comics are based on pointless. But as a Cass fan, I enjoy stories where she wins. And as the best pure martial artist in the DC universe, there are quite a number of characters she can beat. In fact, she could probably defeat any non-powered character other than Batman himself.
Ever since Adam Beechen threw a little possible romance in between Cass and Tim Drake in Robin issues 150-151 (July, August 2006), one of my little funny things to do is draw little doodles of Cass putting a variety of hurtings on a lovestruck Tim. Usually with the sound effect "BEECHEN!" While this would never happen in the comics-- they're just good friends-- it's pretty much how it would play out if the two did tangle. Because there's not a Robin past or present Cass couldn't whup with one hand tired behind her back. Here's how I see the various Robins against Cass:
Dick Grayson? He'd put up the best fight, but how much of one depends on which Dick she fights. Kid Dick from the Golden and Silver Ages would last no longer than any of the later Robins-- mere seconds-- but the older Nightwing Dick could stand toe-to-toe with Cass for a while before she'd inevitably knock him the hell out. After all, this guy substituted for Batman. Still, there's no way we can have a Dick Grayson who's just as powerful a fighter as his mentor-- otherwise why have a Batman in the first place?-- and the comics definitively establish Cass as being a formidable foe for the Caped Crusader himself. Therefore, Dick falls into third place in the Bat-family fighter pantheon behind Batman and Cass.
The caveat to this is if Dick cheats somehow. He's a bit brighter than Cass, and has to be because of his status as substitute-Batman, Batman being the World's Greatest Detective. Cass is no dummy. She's quite smart, actually. But Dick (despite being depicted as a complete idiot due to shaky writing at times) must be close to genius. My reasoning behind this is while Batman would no doubt train his kid army to be as skilled as possible in all aspects of their mission, he must also realize not everyone can excel at everything. He happens to be a polymath, but it's unreasonable for him to expect everyone else to be. Better to have them specialize. He started Dick's training before he figured this out and it's sort of traditional for a Robin to fill that role. So Dick has come farthest towards the all-around standards of his teacher, but Cass is a particularly sharp knife and we do not use such a knife as a hammer and expect it to cut when we need it to later. Cass needs a certain level of detective/brain ability but she's better as a pure fighter.
So Dick has a shot at finding some weakness of Cass's to exploit. Not likely, however.
Jason Todd? Alive or dead, Robin or Red Hood, Cass can take him. He's a rawer form of Grayson, created as a mere substitute and as such, not to be taken completely seriously. I've always questioned the need for a Robin in the first place. Once we grant that first Robin, it takes effort to justify putting a child in danger and really puts Batman's judgement in question (even more so than disguising himself as a bat on the theory that criminals as a whole are a "cowardly, superstitious lot"). Still, most people expect Batman to have a Robin (even if you're not a comic fan, the phrase "Batman and Robin" rings a bell), hence Jason after Dick becomes Nightwing. Hand-me-down characters get harsher beat-downs than originals. That's just how it works.
Tim Drake? Not even a contest. If we posit a Dick Grayson defeat by Cass, and another one with Jason Todd, then obviously this later, lesser Robin has to fall as well. As a youngster, Tim Drake's not even as formed an adventurer as either of the previous Robins. Much less experienced. Yeah, he's become a fine Red Robin, but even now there's no way he could stand against Cass and her lifetime of violence and the resultant special abilities she possesses. In any story where he's even give her the slightest problem can be chalked up to writer error.
Stephanie Brown? Less of a fight than even Drake. In her own comic Cass has been shown to knock Steph out in a casual, off-handed way that smacks of zero effort expended.
Damian Wayne? Despite his having an upbringing similar to Cass's, she eats him alive, and the indigestion she momentarily suffers is the only difficulty she encounters. Tactical genius or not, raised by the League of Assassins and all that, he's still a child with a child's physique. Cass also has him on skill level. If he lives to adulthood, then we might see something amazing, along the lines of Cass versus Batman. But given his personality, what are the chances of that?