Whoops... missed a few days.
Day 18: Which character has the coolest battle theme?
Assuming we're going with trainer themes here, both of Ghetsis' tracks are great. His BW theme really goes well with the importance of your battle with him, and his B2W2 theme goes well with his more power hungry demeanor.
Day 19: Which area has the coolest soundtrack?
The Johto Game Corner in GSC has probably the best soundtrack out of any of the Pokemon games. I could listen to it for hours, and was so happy to find out about the GB Sounds in HGSS. I probably spent way more time playing Voltorb Flip than I should've just to listen to the music, haha.
Day 20: Have you tried any challenge runs? If so, how did they go?
I've wanted to on a few occasions, but I can never remain interested long enough to follow through. My most recent attempt was with a Nuzlocke run on Moon, which I abandoned shortly after the first trial. Maybe if it was an actual feature added to the games, I'd be more likely to complete one.
Day 21: Do you miss the Game Corner? What was your favorite minigame in it?
While my day 19 answer applies here too, I also liked the roulette tables from RSE, especially when it was one of the high roller days. A bit disappointed that it didn't make a return in ORAS, since I would've been content with another game of Voltorb Flip!
Day 22: What's your favorite NPC quote? Why?
"Eek! Did you touch me?"
- Lass Robin, from 20 feet away when I wasn't even looking at her.
Day 23: Do you tend to be either overlevelled or underlevelled when playing?
I'm usually a bit overleveled through the majority of the game, until I reach the Elite Four, where I like to be slightly below the champion's levels. It makes for more interesting fights, and feels like a bigger accomplishment when I win.
Day 24: What's the most annoying status effect for you?
I always flip-flop with my answer on this one, but burn cutting your attack while still injuring you is tough to argue against.