My favorite single-player game is Paper Mario: TTYD. Everything about it feels so magical to me, like the characters, the music, the gameplay, and the humor. I also love the battling style and the strategy involved. I tend to abuse badges often.
I had a lot of nostalgia with it as well, and good lord I still remember how devastating the Shadow Queen was back in Grade 1, even when I had like 50 HP/FP. Then I saw a certain Youtuber fight the bosses with ease, and it changed how I played it forever. =P
My favorite multi-player game (if you count this), is Brawl, since I love the gameplay and Ness/Lucas too much, but if you don't count that, I'll say Mario Party 6. Mario Party 6 was so fantastic to me; the boards were awesome, Orbs were well implemented, mini-games were fun (except for luck ones or ones that are so biased lol) and the Day & Night system was cool too. I had such a great time when my childhood friend brought it over back when I was in Grade 1, and I didn't want it to stop. Then I got it sometime later in the same grade. :p
