1. Percy Jackson and the Olympians: The Percy Jackson series is one that I feel is filled with a ton of different friendships that all add something different to the series. There's the friendship between Percy and Grover which is the one that I think is closest to the typical best friend relationship with how it's the first one we see and is one of Percy's closest friendships that spans through the whole series. There's the friends to lovers one between Percy and Annabeth and there's the one that starts off pretty rough between Percy and Nico that eventually turns out okay. There's a ton of other ones as well and their all pretty well developed even the ones that we are just shown little bits of in the background of the story. I also feel like friendship was also important for the characters being able to be successful in their quests and that without friendship a lot more people would of died.
2. Heroes of Olympus: Being the sequel series to Percy Jackson and the Olympians it comes as no surprise that friendship is once again and important and necessary part in being able to complete and survive the quest in this series. I love the friendships in this series as well just as I did in the first series.
3. Harry Potter: It's been awhile since I've read Harry Potter but I know that the most important relationship in the series has always been the friendship between Harry, Ron and Hermione. Their friendship is the relationship the entire series is built on and since they all have different abilities they are all able to contribute something in how to defeat whatever evil is afoot this year at Hogwarts.
4. Sisterhood of the Traveling Pants: Been awhile since I read this series but I remember the four girls having an extremely close life long friendship that I was always jealous of. I also loved that all four of the girls had such different personalities but ended up getting along and understanding each other as well as they did.
5. Magic Knight Rayearth: I really love the friendship that developed between Hikaru, Umi and Fuu almost right away and I loved how even though they didn't really know each other for very long they started to think of each other as sisters and that they would never betray either of the other two girls for personal gain and that they would die for each other. I really love the girls delightful friendship that developed quickly but had a long lasting impact on all three of the girls.
6. Gallagher Girls: I really love the friendship between the four girls I love how they work together and I love how they're there for each other through bad times and good times. I also just really love that the girls seem like a fun group to be around even in dire circumstances.
7. Uglies: I feel like the friendship between Tally and Shay drives the plot of the Uglies series and I also find it to be a very interesting relationship. Tally and Shay's friendship is a very dysfunctional one that probably isn't healthy considering that the stab each other in the back all the time and the go from loving to hating each other at least once a book but if their friendship didn't exist there wouldn't be a story to tell and their friendship makes the story way more interesting than if one of the romances was the driving relationship of the series.
8. Teen Wolf: I am now going into the TV section of the list because I adore friendships in all mediums. The most important relationship of the show in my opinion is that of the friendship between Scott and Stiles while it doesn't always drive the plot there always there for each other no matter what the other one is going through and seem to be willing to face anything as long as the other is by their side. I love the friendship between Scott and Stiles because I love both of them as characters and I love the way they interact with each other since it's always entertaining to watch but most of all I love how much they love each other and don't seem to want live in a world were the other one isn't in it. I also love the friendships between Alison and Lydia, and Scott and Isaac that are also a part of the show although not given the same amount of importance as Scott and Stiles friendship.
9. Psych: I just really love the friendship between Shawn and Gus and I feel like it's the relationship that gets the most screen time on the show which is something that I am more than okay with since I love to see those two.
10. Revenge: Now while this show clearly isn't about friendship(it's about vengeance obviously) I feel that the emotional heart of the show is the friendship between Emily and Nolan. This friendship is at the beginning the only time that we are able to see Emily being her true self which of course puts a great deal of importance on the relationship but it's the way that it developed from Nolan being Emily's ally that she doesn't really want but reluctantly admits that she needs to being the partner she never expected that she would have. This friendship is by far the most well developed relationship on the show and it's the one relationship that is based on trust rather than deception on the show which is a big part of why it's the heart of the show that would otherwise be heartless. It's also worth noting that when the writers neglect this relationship the show doesn't really pull you in emotionally nearly as much as it should.