The list of tracks I've played and yelled "THIS NEEDS TO BE IN CTGP ASAP" is quite long, so I'll just say the top 4 I've played that should be in
Cityside Nature, because of it's setting, layout, and visuals, a very well done track
Waluigi's Choco Factory, very interesting, unique, and we need more factory tracks in CTGP
Snowflake City, due to its setting, and while it's layout may be a bit confusing, the open-ended design of it is just great.
Desert Cross, a blast to play through, and it's of good quality!
As far as I know, Cityside is actually being suggested for CTGP! Here's hoping it makes it in!