This all depends. Are you speaking critically? Like, what game will likely be chosen by publications and critics as fighting game of the year? Or are you talking in terms of us fans.
If you go by this site, you're gonna get MK. That's a no-brainer.
If you go critically, don't be surprised if MK barely gets a mention. MK has a certain stigma attached to it. The industry is widely populated by Street Fighter/Tekken enthusiasts and elitists that see Mortal Kombat as a joke. Undoubtedly the fighter of the year will be voted as Tekkn v Street Fighter, or Street Fighter v Tekken. That's the way the industry is. The gems like MK will be overlooked and snubbed, and due to popular elitist opinion, SF/Tek will automatically be catapulted to the top.
Not saying i'm not interested in SF v Tek, that's an interesting cross over, but games like Tekken seldom hold my interest beyond 2 weeks. I'm not a fan of 3d fighters, never really was. I only really enjoyed Soul Calibur. Tekken bores the hell out of me. Street Fighter has been the same game for years, even though I did enjoy SF4.
Sorry, my vote goes for MK personally, but critically it's bound to be something else.