Spiky Ring
In some areas brass knuckles are illegal, and rightfully so. But, if you can have a ring with a spike, why would you want brass knuckles? Brass knuckles can't be worn for extended periods of time because they're just way too eye catchy, and you'd be giving yourself a handicap.
A spike ring would however just be a slightly edgy ring.
In the case of self defense, having a spiked ring would significantly improve your chances.
Although you'd risk injuring your finger, the damage you'd be dealing is definitely worth it.