I really like Gurren Lagann, but I've heard that Clannad is pretty cool. I haven't seen High School of the Dead yet, but that looks really awesome. If you want a sure-fire good anime, go for GL. It's not the most original series, but if episode 8 doesn't make you throw your fist up in the air in glorious celebration and defiance of life's challenges while simultaneously making you sob, then I guess you have some empty space in your head for storing spare change or your cell phone or something, because you're obviously missing the emotional part of your brain.
Skip K-ON though. It's a series about normal, lolicon, highschool, bff girls who start a band and perform in the various school talent shows. That's it. There's no significant conflict, no well developed characters, no intense action scenes, no deep psychological/philosophical/spiritual challenges, no romantic interest, not even any good fan-service scenes. It's a series about hyper-cute anime girls who do hyper-cute anime things and live happily ever after. If you're a 15 year-old girl who is obsessed with ANY cartoon that comes out of Japan and consume nothing but Mtn Dew, pocky, candy, and cream soda, and don't want to have your mind challenged in any way, shape, or form, watch this series. If not, skip it.