Hmm, in terms of causing the greatest good for all...
I'd have to go with a universal sense of right/wrong. Basically what the Christian concept of a conscience is supposed to do, but utterly fails at.
Without amping the feeling of guilt up to mind control levels, we'd still have crime and possibly war, but the frequency and intensity for both should drop drastically.
Its not all that much fun when talking about "superpowers", but most fun options are likely to cause a fair amount of chaos.
Although superintelligence wouldn't be too bad, just shift the IQ bell curve 50 or so points above its current average.