I've been carefully monitoring the app updates on my phone to see which apps I use every day have watch extensions. Honestly, there are very few that I feel need to be on the watch on day one, if at all. I suppose there is a compromise between having the app available and disabling notifications, but I want to be very careful not to make the watch an iPhone clone. Only apps that offer valuable information in a quick glance are going to make it onto my watch as a result.
Apps with a passbook component probably have the best chance of being first, and MLB At Bat for game notifications.
1Password intrigues me, but I'm not sure what kind of utility it will offer on the watch.