Some of my friends at school were talking about this, and I decided mine was Super Mario Maker on the basis that it was the only video game released in 2015 that I've actually played.
For games not released in 2015, I'd have to say Dynasty Warriors Gundam Reborn.
Yes, I know, a Dynasty Warriors game? Based on an anime license? Well, it's obviously not a masterpiece of video game design, bur I'm in my senior year of high school now, and my workload has increased greatly so I don't have as much time for serious gaming. So this has been a nice relaxation game for me. 100+ hours on a game I bought in August.

Plus, I've been getting into gundam a lot recently, as evidenced by my C̶h̶a̶r̶ ̶A̶z̶n̶a̶b̶l̶e̶ Quattro Bajeena avatar.
I guess there's also Marvel Vs. Capcom 2. I first played it on my dad's Dreamcast in the early 2000's. The last time I saw him before his death was my 11th birthday, where we went to an arcade in the mall (which no longer exists anymore) and played it. I then got it on PSN, but that account has been dead for years now. So I didn't get to play it again until I finally caved and payed the $40 for the Dreamcast version. It was definitely worth it.