I know there are already several third party Strava apps... but still we wanted to create our own!
So here it is Kliva:
Kliva | Windows Phone Apps+Games Store (United States)
It currently lacks the option to actually track routes. I know a big let down, but because we opted to go for the new Universal app approach with wp8.1 WinRT sdk, this is currently not possible. Should be fixed with wp10 SDK - so fingers crossed.
But still I hope you would give it a try and maybe comment below on what Strava client you like on Windows Phone and why, because that way we can maybe better ours :)
So suggestions are welcome.