That would be a cool app and I bet there are already a thousand already out there. Which one is the best and the simplest?

Touch and see the price of BTC, nothing else.

Found this answer on reddit, no app required:

If you're a bitcoin fan, you can get the updated price of Bitcoin on your watchface as complication. Here's what you do.

Go to the stocks app on your iPhone and add a new stock quote by hitting the small plus on the upper corner. Then paste this in:

BTCUSD=X

That's it. Then go to your Apple Watch app on your phone and find the stock settings and choose the default stock quote to be the Bitcoin thing you just added.

On your watch, the price of Bitcoin will now be shown if you have a watch face with the stock complication active

Voila.

Link: https://www.reddit.com/r/AppleWatch/comments/3bard0/how_to_get_the_price_of_bitcoin_as_a_complication/