I think Argentina might be the most vulnerable of the big sides - wouldn't surprise me if any of the other three came out of this group. Croatia have a lot of talent but Iceland finished ahead of them in qualifying. Nigeria are a good team too. I went with Argentina and Iceland.
If you're looking for upsets, this is the group. It's easy to pick Argentina, but it could backfire. Nigeria may be the best of the African nations and Croatia sometimes plays great and sometimes poorly. Iceland is very hard to figure. It may have been a one time event what happened at Euro 16. I'll go with Argentina and Croatia, but look for a real close finish here.