First off it is good that Fallout 4, Deus Ex: Mankind Divided, as well as Mass Effect 4 would be competing against each other at E3 this year. As WRPG fans, this is only good for us. Despite what you may think about DAI, you have to understand that DAI had not competition in the WRPG department and at release, many reviewers were lenient on DAI for the fact that it was the first current gen WRPG out of the gate. With games like Fallout 4, Deus Ex, and Mass Effect, the games would be viewed in a more stricter way.
As for who I think will win E3 Best RPG, I will have to say Mass Effect 4. Fallout 4 might be the more popular game, but Mass Effect 4 just has a level of secrecy that is ready to explode at the EA press conference. People will be blown away by the graphics, character dialog, and gameplay features that are announced. With Fallout 4 and Deus Ex: Mankind Divided, I believe that these two games will just be "prettier" versions of Fallout 3 and Deus Ex: Human Revolution whereas Mass Effect would almost feel like a new game, TOTALLY different from what we saw with Mass Effect 3. This alone will give ME4 the edge and will steal the show in terms of excitement.