For me, both of the situation hurts but if we're talking about the degree of pain, I believe the latter is more painful. Unrequited love hurts and as the other comment said, you know you shouldn't expect anything from the other person since the feeling is one-sided though it may or may not be confessed. Since they already know that it can go both ways, they still have time to decide if they'll give up on their feelings or not, unlike for those who have partners' who fell out of love, they'll be in a state where they would fight for their feelings and for their partner to stay for all the times they've shared and have been together. It's just that experiencing unrequited love, I guess it's easier to move on(my opinion but still depends on a person) since there was really no concrete relationship or label (except for those with having friendship and such, still depends on the person). For having your partner fall out of love is more painful for the reason it'll agonize you and will question yourself where you lacked or what you did wrong. Also, the relationship and memories you've experienced together would seem trivial and easily discarded/disregarded, thus asking yourself if you were deluding yourself for believing that your love would last forever. It will be hard to accept and hard to move on from since you've been through a lot together and it's hard to get rid of your feelings for that person. But then again it still depends on the person if which is more painful since we can't really invalidate the feelings of each person experiencing such situations. We really can't measure the pain one will feel when in that kind of situation.
What hurts more(based on your opinion)? Unrequited love or having your partner(lover) fallen out of love?