Fast is a combination of fast startup time, fast end time, and how long the hitbox is out, and overall time. DAir is just plain long. Only move that if you ledge hop wd off double jump, you can't waveland/get a second aerial out.
You could argue that his jabs are his fastest move, but of course they are, they're jabs. It all depends on what you want. If you're on the ground, the jab is definitely the fastest to hit. In the air, i'm not sure whether nair or uair comes out first, but uair does end hella quicker. The bair though...not sure how fast that is in comparison to the uair. It also auto cancels.
edit: yeah dash attack is actually pretty nice if you use it right. You can combo with it against floaties, and dropzone with it against some other characters, and it's fast as mentioned. If it's going to put the character off the stage I use it pretty often if I'm not sure I can pull something else out in time. You do have to be careful with it though, because of that stupid tap of the second hit. Why can't we have Samus' DA D: