Everyone is just going to tell you to watch their favorites. But to be honest, you need to watch something will be appeal to you. If you like action, fantasy, romance, blah blah. Watch stuff like that. It depends on what genre you are looking for. I'll list some genres and some good shows in those genres. Keep in mind, some can be in multiple categories, and there are other categories I didn't list. It would be easier if you mentioned what type you were looking for!
Action: Shingeki no Kyojin (currently airing), Code Geass, Black Laggon, Fate/Stay Night(Fate/Zero as well).
Romance/Drama: Sakurasou no Pet na Kanojo, Angel Beats!, Toradora!, Kaichou wa Maid-sama, The World God Only Knows.
Fantasy/Supernatural: Shakugan no Shana, Zero no Tsukaima, Kaze no Stigma, Sword Art Online, Devil Survivor 2 The Animation.
Slice of Life/School Life: Love Live! School Idol Project, Boku wa Tomodachi ga Sukunai, Koi to Senkyo to Chocolate, Minami-ke.
Mystery/Psychological: Higurashi no Naku Koro Ni, Kami-sama no memo-chou, Hyouka, Danganronpa (currently airing), Mawaru Penguindrum (might mess with your head a bit).
Comedy: Hataraku Maou-sama, Blood Lad (currently airing), Yuru Yuri, Tamako Market.