I have recently received many wonderful recommendations for Young Adult Fantasy series and here they are: 

My first recommendation is the Fins trilogy which begins with Forgive My Fins by Tera Lynn Childs, tells the story of Lily. Lily is half-mermaid and half-human. She lives on land and is afraid to tell her crush that she is the future ruler of the underwater kingdom of Thalassinia. 

Forgive My Fins is followed by Fins are forever, and it ends with Just for Fins. 

The novel Need by Carrie Jones, tells the story Zara White. Zara is a young girl obsessed with phobias. She  is sent to live in Maine after her stepfather passed away. Zara is afraid that someone is stalking her. Her stalker leaves behind trails of gold dust... because he is a Pixie and Zara will soon find out why he has been trailing her. The novel is full of suspense and romance.  

Need is followed by Captive, Entice and ends with Endure.

Watersmeet  by Ellen Jensen Abbott. Watersmeet tells the story of Abisina. She is viewed as an outcast in her village because she has very distinctive eye and skin color. Her world is one where people live under two extremes, one being prejudice and the other being loyalty and duty.  Absina’s life changes when a new leader comes into her village. She is forced to flee in search of a father she has never known and for new hope for herself  in a place called Watersmeet. 

Watersmeet is followed by The Centaurs Daughter and ends with The Keeper. 

Daughter of the Centaurs by Kate Klimo, tells the story of Malora. Malora is a very strong main character. She is a hunter and horse wrangler. Malora’s people are killed and she is left as the last human in the world trying to survive. Malora is captured by centaurs and must choose whether the comforts provided by the centaurs are worth losing herself. 

Daughter of the Centaurs is followed by A Gathering of Wings. 

The novel of Of Poseidon  is the  first book in the The Syrena Legacy series by Anna Banks.  Prince Galen of Syrena, is sent to land to find a girl that communicates with fish. That girl is, Emma, who is on vacation. When Prince Galen finds Emma he immediately feels a connection to her. However, it is hard for Galen to convince Emma about the strength power, but he has to because she holds the key to his kingdom…

Of Poseidon is followed by Of Triton and concludes in Of Neptune.

Finnikin of the Rock  is the first novel in the Lumatere Chronicles by Melina Marchetta. When Finnikin was a child the Lumatere family was murdered and replaced by an imposter. Finnikin must now join forces with a girl named, Evanjalin, a fellow exile who believes her dreams are premonitions of a better future. She believes that her dreams will lead them to  the last surviving royal child who can help the exiles from a horrible fate. 

Finnikin of the Rock is followed by Froi of exiles and Quintana of Charyn. 

Stolen Songbird   is the first novel in The Malediction Trilogy by Danielle L. Jensen,  Stolen Songbird tells the intriguing tale of Cecile de Troyes  who is kidnapped by trolls and taken to Trollus.  The trolls believe that  Cecile is a witch with the power to break a curse that a  witch put on them  placing them beneath a mountain for five centuries. The trolls have been under the mountain for so long that many believe their existence is just a myth. 

Cecile plans to escape from her kidnappers as soon as she gets the opportunity but as time passes she begins to fall for the prince of Trollus. She is bonded to marry him.  In Trollus Cecile begins to make friends and sympathize with the plight of the half human half troll creatures that enslaved to the full blooded trolls. Cecile finds that she can become a strong leader in Trollus and help the brewing rebellion...

Stolen Songbird is followed by The Songbird Overture. The third an final book is untitled as of yet.

Lastly, the novel The False Princess by Eilis O'Neal is the tale of Nalia. Nalia has been raised as the Princess of  Thorvaldor for sixteen years. Until one day she learns that her real name is Sinda and she is part of a political plot to change the future of her country. 

Other underrated series you may enjoy: Three Underrated Young Adult Fantasy Series.