I think Liar Game. Of course I can think of many others that made me want to keep watching but these things are often a matter of personal taste. Liar Game seems to be popular with almost anyone so I think it's a safe choice. The trickier part is drama #2 and #3, to make sure your friends are properly hooked, because of course you want them to keep watching after the first one... I think by then you can guide them based on their reactions to Liar Game.
I loved the first j-drama that I watched, Rich Man, Poor Woman... but that's just me; many people didn't like it at all. Watching it made me want more, that's the main point, however I found it hard to go on. The second drama I tried was Hanakimi because people in my Japanese class recommended it; however I did not even finish the first episode, I just didn't like it. (It's so popular I might give it another try someday though.) Then I tried one called 100 proposals or something; didn't watch more than one episode. My third try was 14 sai no haha, and I did watch a few episodes but didn't get hooked. By then I was ready to give up on trying to watch j-drama (my main reason for wanting to at first was the language). What saved me was the fact that I realized I wanted to see more of Iura Arata from RMPW, so I did, and I'm so glad I did! I then watched Hana yori dango (with all the story's flaws, it was strangely addicitve even to a 30-something) and the rest is history... :)
