http://www.ign.com/videos/2013/10/29/assassins-creed-4-black-flag-video-review
I originally wanted Killzone first but after watching IGN's video review of Assassin's Creed 4 on PS4 I am moving it to the #1 spot in my gamefly queue. I still want to play Killzone but I also will be playing COD Ghosts and Battlefield 4 online with friends. Assassin's Creed will be a nice change up from the FPS games. My opinion might change as more PS4 reviews start to come out but right now I am shooting for AC4. 
I have mentioned in other threads that I am kind of over the Assassin's Creed series. I rented AC3 but sent it back after about 4 or 5 hours of playtime. My big problem with that game was that even at the 5 hour point I still felt like I was in a tutorial. Instead of giving me freedom to do what I want they were still holding my hand and telling me what to do. From the IGN review it sounds like they fixed that problem and opened it up right from the start.
Now I just have to finish up GTA V and get it sent back in time to be in the first wave of PS4 launch games from gamefly. It might not matter which game is first on my list because they struggle with inventory on brand new releases. I'm going to make sure I have an open spot in my queue and only PS4 games on my list. Then it's just a wait and see game to find out what they will send me.