Definitely Paper Mario 2: The Thousand Year Door. Even though I never actually owned it, a friend loaned it to me for about a month back in 2005 and I really enjoyed it. I can still remember laughing at the antics of many characters, such as Lord Crump, Luigi's Tale, Doopliss and the Shadow Sirens, Goombella, Bowser and Kammy Koopa and Princess Peach with TEC.