I second The Witcher 3. That game goes on for months while still being very enjoyable. I'd recommend Diablo III only if you have other people to play with as the game gets somewhat stale when played alone (that's how I feel, I know some friends who disagree with me lol)
I finished Dues Ex: Mankind Divided days after it came out because I'm shamelessly addicted to stealth games. This one is kind of easy if you're looking for a challenge, but the story is wonderful and mirrors life in a spooky way (like a very mild Black Mirror kind of way). I'd recommend this because it is fun, but its not a game that takes a lot of time to complete like Witcher/Diablo.
I'm also curious about the various Assassin's Creeds (probably because there are a slew out there) along with Shadow of Mordor (huuuuge Tolkien nerd here)
For Assassin's Creed, I'm of the minority that really enjoyed AC III. It had ship battles in it but it wasn't pirate themed, it was about the Native American culture. I still think AC II: Brotherhood was the best one in the series though, and Black Flag wasn't too bad. Honestly, if you're willing to look past the somewhat long loading times, the only criticism these games get is being repetitive. So my advice is to try one or two games only, that way they wouldn't get too repetitive for you. If you try most of the series, like I did, you'll see they're extremely repetitive and you'll get bored very quickly.
Shadow of Mordor I definitely recommend. They don't really follow any of the books, but they are somewhat true to the lore. The game play is very enjoyable; you can play it stealthy, aggressive, or approach it more tactically. The dynamic difficulty is fun, challenging, but slightly forgiving.
So, out of those, in order I'd recommend Witcher first, then Mordor, then Dues Ex
As for horror games, I didn't like RE6 so I'm skeptical about RE7, I'll have to wait and see the reviews. I really, really loved Evil Within though. Its very close to the old RE games in terms of atmosphere but its controls are much better. Its a bit gross though, lol. Also, I should note that I don't normally enjoy horror. Evil Within was a exception though.
Dishonored 2 comes in two days and I'm really excited for it. If you like stealth games you might want to look into it too.