I can't remember where, but I read a study on helmets and their ability to withstand impact and International and Charles Owen helmets came up as two of the best.
One important thing to remember too is, no matter how good of a helmet you buy, if it doesn't fit your particular head well, it won't do you any good. It should be snug, but comportable. When you put it firmly on your head and it isn't buckled under your chin, you should be able to bend forward, looking at your knees and it won't fly off your head. When buckled up and properly adjusted, when you rock it from front to back, it should be snug enough that your eyebrows move. You should not be able to rock it much from side to side. Some helmets are very round and some are more oblong. Try some on before you buy.
Don't buy used, you never know if someone fell on their head or dropped it on a hard surface which destroys the integrity of a helmet.