Which online challenge would anyone say be the one to try the most
Which is worth it
Both the Website and Photography Challenge are pretty good options. The Website can be a big benefit to your team even if it does not win, and the Photograph does not require a lot of time to be put into it.
If you want to go a step farther, the Promote video can even be a part of your website, and if you have the software and anyone who likes playing in CAD, the field will be a bit more open since this will be the first year with CAD submissions focused around V5. May even be worth going back through previous winning submissions and seeing if anything could be updated to work well with V5.