Title says it all. An example would be Unwavering Emotions from B/W, or Dialga and Palkia's theme from D/P.
As sumwun mentioned, there aren't many battle tracks that feature a piano. However, many other noteworthy songs have piano/keyboard as one of the main instruments.
*Songs with this mark are not primarily piano-based, but the piano is noticeable and has a significant purpose in the song.
Please leave a comment if you think a song is missing, I will add it in.