Hmm...
Everyone gave some essentials so I'll try to list some not so mainstream anime's that you may like if you enjoyed "Attack on Titan" that much.
"Aldnoah.Zero" - A very interesting take on mech-based anime (Giant Robot Anime) that is similar to the Gundam Series, but way different in storyline in my opinion. The story is fantastic, it's thrilling to watch the main protagonist do what he does, and the soundtrack for the anime is fantastic in my opinion. It's very much an underdog story to a large degree, but very satisfying.
"Knights & Magic" - A fun short anime where the protagonist is reincarnated into a world of magic where magic mechs are the main source of military might, and using his otherworldly knowledge the protagonist advances the technological developments to the extreme. It's fun, exciting, and has a lot of amusing moments. It's a little adventure and the episodes feel longer than what they are.
"Sword Art Online" - I'm sure you've heard or possibly watched this anime already, but I'll list it anyways. Essentially it's an anime where a large group of people get stuck inside of a virtual reality game via the fact that if their full dive helmets (The tech that allows them to fully integrate into the digital world) are removed, their brain dies, thus killing them. It's a dog eat dog story where surviving is the goal, however it of course has adventure, action, comedy, and passion full to the brim, and also has like 4 seasons and an offshoot short anime. All in all, it's great anime with a lot of episodes so far. Around 24-26 episodes per season so plenty to keep you entertained.
"Log Horizon" - A different take on the Sword Art Online story. Instead of being forcibly stuck in a game, a large group of players of a game are not stuck in the game, but the game has become their reality instead. So, they are stuck, except it's become their reality, lol. It's weird, but good, and there are like 2-3 seasons with another one on the way. It's action packed, has comedy, drama, a little bit of romance here and there, some sweet kind moments, and great sound track and some thoughtful deep moments.
Just a few anime that are off the beaten path of the mainstream, and are very enjoyable in and of themselves. I know of several others, but that's all I feel like listing right now, lol.