Well for me personally, the announcements I always enjoy the most are the ones that come completely out of left field. I mean sure, it's always cool to get sequels to IP you love, but unless it's something that's been dormant for a while, you pretty much know that's gonna happen sooner or later. E3, for me, is about shock and awe. Show me something I haven't seen yet.
That said, if we're talking existing IP, what I want to see most is Cyberpunk 2077. I know that's technically been announced already, but that was ages ago, and we've not actually seen or heard anything tangible from the game, so a proper E3 unveil at this point would basically be a real (re-)introduction.