monarch - church, pope was the main challenge, especially after 1000. Another challenge could be rebellious aristocracy (king John in England, French kings during the 100yrs was were challenged by dukes of Burgundy, Elizabeth I was challenged by Catholic nobles etc etc.). Benefits = they were sovereign rulers with many open possibilities (needed always one thing ..good councelors).
clergy - big challenge: heretics, hereticism/benefits: probably the biggest and the most influential organization of Middle Ages with big influence in all areas of life.
nobles - benefits: privileges, always possibility to rise higher (through marriage, purchase of lands, participation in war, religious policy or relations with the king)/ challenges: rebellions of serfs, political aliances against them, accusation from the church (because of marriage policy or religious policy)
knights: - benefits: participation in wars can bring them fame and money/challenges: poverty
artisans: - benefits: if they get contract from royal court or papacy they can be really well off, famous and prestigious/ challenges: some of them always on the edge of heresy, arts were always part of politics (it can be dangerous).
merchants: - benefits: they can become influential and powerful if their wealth is expected to help to popes, kings, aristocracy/challenges: dangerous especially if they are Jews
Serfs: - benefits: in certain areas of life they are relatively free/challenges: poverty, obedience (sometimes too much).
Explanation:
All these answers are correct but do not forget that things were changing depending on time (historical period) and country (there were many different countries in medieval Europe).