Some people probably mentioned some of those already but I'm just seconding them !
Try basically anything by SM artists, they are the "royalty" of Kpop to me <3
SNSD : Into the New World, Girls' Generation, Gee, Tell me you wish, Oh!, and RunDevilRun .. Their album tracks are pretty good too, I recommend that you listen to the Oh! album and Genie mini-album
SHINee (my favs <3) : Replay, Love Like Oxygen, Ring Ding Dong, Romeo+Juliette, Last Gift, Y.O.U, Jojo, and The Name I Loved (Leader Onew's solo)
f(x) : Chu~, You Are My Destiny (Luna & Krystal duet), and all the songs on their new mini NU ABO
Super Junior: Marry U, Bonamana, Cooking?Cooking! (they have many good songs but I don't have all day XD)
MBLAQ : Oh Yeah, Y, G.O.O.D Luv, One Better Day (watch their mvs and lives too, these guys are smokin hot !)
2AM : I Did Wrong
CNBLUE : I'm a Loner, LOVE, Sweet Holiday, Love Light
There's more .. but I can't think any now D: