Good to see some people still discovering Shoko, though she has been around a while now.
The latest cover cds include several songs I already knew as I like older Japanese music (70s/80s particularly). It's probably impossible to improve on the original versions of Friends (フレンズ), Diamonds and also the songs by Yoko Minamino (吐息でネット), Yukiko Okada (くちびるNETWORK) and Yamashita Kumiko (赤道小町ドキッ) all absolute classics, so Shoko is giving herself a tough task there. I think her voice fits best with the Yoko Minamino song, and it's interesting she mentions her as being someone she would do a duet with. I didn't realise she was a fan of that song and singer.
Shoko's voice also fits nicely with the Seiko song (蒼いフォトグラフ), a singer that it's well known Shoko idolises (and rightly too), though I think Seiko did many songs that were better. A song I didn't know so well was the Coco song (はんぶん不思議), though I must have heard it before, it's not bad particularly the verse and the riff at the start. Shoko seems to really get into that song as well.
Overall then I definitely think the smoother pop songs suit Shoko best, though what Japanese sometimes call rock would probably be called pop often in the West anyway. She always gives very enthusiastic and energetic performances of 'rock' styled songs and is very professional, but I feel her heart can be more in the pop music and maybe one of those videos above expresses this as well. The many good songs from Japan in the 70s and 80s will definitely not grow old and really it's what got me to discover Shoko in the first place back in 2006 or 2007 with her singing along to Seiko songs in youtube clips from that pink room program she did.