O-Parts Hunter/666Satan: This is one of my favorite action series manga and I'm currently trying to get all 19 volumes of it. The biggest problem this series has is that there are heavy similarities between this and Naruto (people sent him letters saying he was a fraud), this is because the creator of O-Parts Hunter is the twin brother of the creator of Naruto and he mentions that they had the same influences growing up. It doesn't excuse everything, like the art style being almost exactly the same, but it forgives some of the other similarities.
Liar game: This is kinda cheating because it was made into live action movies but movies came only hold so such. Seeing all the different games come to life would be amazing. It has a Light/Lelouch type main character but a lot less egotistical which makes him much more likable. And some of the games they play are just a good mind trip.
Tenjou Tenge & Berserk...the whole thing