Loaded question. In all honesty you are putting what MOST people consider three of Stern's best against what MOST people consider one of Stern's weakest. I am heavily biased to Mustang as I love the theme, think it is one of the best looking games ever, and really enjoy the unique layout. I will also freely admit that I am still in the honeymoon stage with mine, but I can't see my opinion of it diminishing at this point. I love everything about it, but I can understand why many do not enjoy it as much.
My personal opinion is Mustang walks all over AC/DC. No one will agree with me, but it's my opinion, LOL! That game just doesn't grab me all that much even though I enjoy the music and love Ritchie games in general. I just do not understand that love it gets. But that's just me.
TWD...that's a closer one. I am not all that familiar with the rules, but I know it is very deep with some pretty interesting/unique stuff going on. I love the theme, but do not find it to be a very attractive looking machine...just very...brownish, LOL. That's not really a knock on the game as it hits the theme well, but compared to Mustang it just isn't as nice looking in my eyes. I also kind of hate how the well walker looks...just a big turn off for some reason. I mean, I love zombies and all, but that dude is just ugly, and not in a good way. It is a unique layout that is a lot of fun to shoot, and the translate/cabinet are great looking. Even though it may not tie directly to the show's characters well, it hits the overall zombie outbreak feel pretty well. Overall, great game and very high on my wish list, but not up there with Mustang to me. That's subject to change with more plays and understanding of the rules though.
Metallica...that's pretty much a tie for me and if I'm being 100% honest I would probably have to say Metallica is the better game once my honeymoon with Mustang wears off. Metallica just has that something special where everything comes together perfectly. I love the theme and music, the artwork is possibly the best ever, the layout is nearly perfect, and the software is awesome. It is definitely #1 on my wish list and once finances allow, I will have one. Mustang is definitely a "lighter/brighter" game while Metallica is more "heavy/dark"...if that makes any sense? Metallica is more intense while Mustang is easier going...that might be a better way to put it (although you can make Mustang feel more intense depending on your custom music choices for sure). They are a pretty good contrast to each other actually, but both can be very fast and adrenaline pumping.
Again, these are only my opinions and I realize not too many people would agree with me (especially about AC/DC).