I would say the best attitude is Assertive. This way you can stand up for yourself without coming off as completely rude. Also, it uses the word "compromise", which is an agreement or settlement, which is great. Passive-aggressive is a good attitude as well, but it says you'd sabotage the other, which isn't the best attitude then. As for passive, you'd be giving in too easily and wouldn't stand up for yourself, which is not a good attitude for resolving conflicts since you would then be losing everything.
The hardest attitude to deal with would be aggressive since they don't give in and will do all they can to win.
The hardest attitude to keep if you are in a conflict would probably be passive since you may end up getting aggressive, but it really depends on the individual.