As of today, it's Halo 5: Guardians. I thought the campaign was good, and the MP is probably my favorite so far this gen. This is the game I've wake up early to play before work.
But, I'm also playing Rise of the Tomb Raider, and have a strong feeling that will sit right next to Halo. I've also yet to play Fallout 4, I hope to be able to put some time in closer to New Years, but based on the streams I've watched and how much I liked 3 and NV, I'm pretty sure it's one I'll enjoy. I have a feeling those 3 games (in no particular order as of now) will round out my top 3 for 2015.
I've seen lots of mentions of Witcher 3 - After 25 hours I stopped playing with no desire to go back. Fun for awhile, but I got bored. Liked W2 better.
Games I see being mentioned that I've yet to play or won't play this year:
Batman Arkham Knight (AC was a let down, likely won't purchase for awhile)
Bloodborne (No PS4, not a fan of Souls gameplay)
Rocket League (No PS4)
MGSV (Enjoyed GZ, but if I ever do pick it up, it won't be for awhile)
Ori (would still like to, but not the biggest fan of Metroidvania style games, may get next year)
Star Wars Battlefront (Beta was fun, no desire to purchase or play again)
Call of Duty BO3 (Beta was ok, watched streams of campaign, won't be purchasing)