As much as I love a good anime series, my heart really lies with manga. I figure it's just because I've always been more of a reader. The only downside to manga vs. anime is that manga books are so expensive :'( Anyway, here's my top 5 favorite!
Ranma 1/2 - This is the first series I read! At the time I didn't even know it was considered manga, or even what manga was - I just liked reading comic books! It's sort of a martial arts comedy, with a little fantasy thrown in. All the characters are so fun and wacky, it's hard to ever get bored of it. My favorite character is Ryoga. He has a terrible sense of direction (like me), and transforms into an adorable little pig :) I also like how you don't necessarily have to read the books in order, and they're all just little story arcs here and there.
Fruits Basket - This was the first series I read once I finally figured out that manga was a thing. A friend of mine already had the first 4 volumes, so I borrowed them from her and devoured them in a day. After that I read them religiously as they were released until the end of the series. Fruits Basket is a drama/comedy/fantasy that revolves around the story of the Chinese zodiac. I'm definitely Team Kyo <3 I also like the anime of this series, but not nearly as much. Everything is in a different order, I wasn't feeling the voice actors, and everything was just kinda wonky.
Nana - Even though I love it so much, I can't help getting sad whenever I think about Nana. In 2010 the author (or manga-ka if we want to be technical about it) got sick and stopped writing the series. Since the hiatus was unanticipated, the last chapter ended on SUCH a cliff hanger, and now I fear we'll never know what happens. She's reportedly said that she does plan to finish the series, but after almost five years and no real concrete news it's hard to stay positive. Anyway, I first read Nana in Shojo Beat magazine, which isn't a published magazine anymore, but was exactly like Shonen Jump with shojo manga instead. It was really great. The series is very dramatic and not normally the type of thing I read, but all the characters are so unique while still feeling very realistic which reeled me in. They all make stupid mistakes, but it's hard to hate them for it because it all just feels so human. Growing up is hard, and we all make mistakes sometimes!
Pita-Ten - This is a really short series (8 volumes) but manages to pack a lot of story into them. It's about a boy who befriends an angel named Misha and a demon named Shia...except he doesn't realize what they are right away. It's weird and cute. I got to see the author, Koge-Donbo, at Anime Expo one year. It was awesome!
Fullmetal Alchemist - Because obviously. I've already talked about this one a million times so I'll save you the trouble, hahah.
A few runner ups are Yotsuba&!, Hellsing, Girl Got Game, and One Piece! Maybe I'll write about those some other time. Do you have a favorite manga series?