Donkey Kong's back, and his custom moves are...honestly, compared to Fox and Ganondorf, really underwhelming. He's got some unique options though. I couldn't see too many of these being moves you'd integrate into a normal moveset, but I could see someone perhaps making a custom moveset with one or two of the moves to swap to if they were having some issues defeating an opponent with the normal one.
-Standard B-
Custom 2: Donkey Kong charges up his punch faster (at the cost of some power) and has electric effects. Could be useful if you want more speed instead of power.
Custom 3: Shown in a pic of the day. Releases wind that pushes an opponent back with a fully charged punch. Still does damage if the punch connects. Can be very useful for edgegaurding
-Side B-
Custom 2: Jumps in the air and then does a headslam. Could be a good option against airborn opponents maybe?
Custom 3: The same move as usual, but slower and more powerful, and leaves the opponent in the ground longer. Has super armor, which is cool.
-Up B-
Custom 2: Does no damage, but gains a lot of height from it, especially in comparison to the normal jump. Recovery's kinda easy in Smash 4, but if you're still having trouble with DK, use this.
Custom 3: Does damage on the first hit and more at the end, instead of a constant stream like normal. Seems to suck the opponent in between those two hits too. Might be useful.
-Down B-
Custom 2: Electric effects. Has good knockback.
Custom 3: Fire effects. Not much else remarkable about it.
Honestly, these are just really underwhelming. I kinda think maybe Sakurai thought DK was OP and didn't give him that great of a variety of custom moves? I donno. But these just aren't very overpowered like some of the other customs seem to be.
Video of the Custom 2s:
https://www.youtube.com/watch?v=umNBc5gMQpw&list=UU_2qLJkKXkaqk3e8JXcQNcw
Video of the Custom 3s:
https://www.youtube.com/watch?v=cFnGSwfrTfs&index=2&list=UU_2qLJkKXkaqk3e8JXcQNcw
Note that the videos are crappy offscreen recordings, are played against a CPU, and feature me mostly spamming the custom attacks to see what all they can do.