I voted Pokemon, because that is the game that Nintendo will try to push the most. However, I don't think that will be the thing that stands out the most in their presentation.
Instead I think what will stand out is the huge amount of quality content coming to Switch. Their E3 "Direct" will be a lot like their September 2018 Direct only better. It will have at least as much content, but the third party games will be even more impressive. Most the of the third party games will either be exclusives or multi-plat games that haven't been released yet on any system. On top of that 2019 is going to have a better first party line-up than last year. Put all of that together and you are going to see their best "Direct" yet.