So, as some of you may know, I'm currently in the process of posting a Shadow/Peach fanfic to Fanfiction.net. What you may not know, however, is that mine wasn't the first. Before that, another user posted a fic titled Bittersweet about Shadow and Peach befriending each other during the Winter Olympics. Sadly, that one - as riddled with spelling and grammar errors as it was - was deleted recently, so now I'm the only game in town when it comes to Shadow/Peach fics...as far as I know, anyhow.

https://www.fanfiction.net/s/13946181/1/My-Baby-She-Likes-to-Rock

And here's another Sonic/Mario crossover fic I did, which features Shadow x Peach (Shadeach? Peachadow?). While not the first fic to pair these two, I'd say this is quite possibly the best one yet.