The Uglies series by Scott Westerfeld - This was one of the first book series I ever fell in love with! I was in, like, 6th grade the first time I read this, and I still think it's just as brilliant as I did then.
The Vampire Academy series by Richelle Mead - I have said (exclaimed) this so so so many times, but I will honestly never stop. This series is my #1, my ride-or-die, my all-time favorite. The characters, the worldbuilding, the writing. Everything about it owns my heart.
The Pushing the Limits series by Katie McGarry - Katie's books are just amazing in every single way and I think everyone should experience these beauties.
The Mortal Instruments series by Cassandra Clare - Despite what the movie and TV show make it seem like, these books are actually amazing. The world is engrossing and the characters are addicting.
The Shatter Me series by Tahereh Mafi - Between the unique writing and the wonderful romantic tension, this series had no problem winning my fangirl squees.
The If I Stay series by Gayle Forman - I'm not sure if this really counts as a series, but there are two books, so I say it does. Anyway, these books are emotional and adorable and absolutely captivating.
The Caster Chronicles by Kami Garcia and Margaret Stohl - There are 4 novels in this series, all of which are pretty thick. It seems like a hefty journey to embark on, but it's so worth it. The lush southern gothic setting and the cast of characters we get to know throughout the series make it all worth it.
The Infernal Devices series by Cassandra Clare - What can I say? Clare knows how to write 'em.
The Delirium series by Lauren Oliver - Beautiful and heartbreaking and poignant and unique. I recommend this series to literally all my friends.
The Hunger Games series by Suzanne Collins - I mean...it's The Hunger Games....
Are any of these series on your list of favorites? What are some other series you want to gush about all day?