I gave my merchant Friendly and Eloquent, both of which I hoped would help him make friends. But his fatal flaw - Lascivious - was a real nuisance because he was very unhappy if he didn't get kissed twice a day at least. As soon as I got the chance to get a legendary trait for someone, he was the one I chose. I haven't tried all the fatal flaws yet, but as others have said, Hubris is a hard one. Glutton is challenging too because they need to eat twice as often, and never seem to be totally full. Of the fatal flaws I've tried, Drunkard and Compulsive Gambler seem to me to be the easiest ones. All they need is a drink or a bet twice a day to keep them happy, and neither of those needs takes a lot of time to fulfill. I've also tried giving Puny to characters like the priest or doctor that don't need to get into sword fighting. From what I can tell so far, it isn't causing them any problems.