What type of characters do you find the most challenging when it comes to animating? Cartoon Characters, Live Action Characters, or Sprite Characters?

Adam Powell
Overall, Live Action. But that doesn't mean all Live Action work will be harder than the rest.
Along with the type of character, how challenging one animation will be is also defined by: the type of movement.
Do you want a character walking sideways?
This may be significantly easier with sprite characters, for example Yume Nikki already provides sprites for Madotsuki doing this. That's not so far from being a copypaste job.
Now, do you want a character running towards the camera?
Yume Nikki provides no sprites for this, so Image by Image work with a sprite character is required.
Madotsuki doing some "front running" was more of a challange to animate than this Video Cropping with Regil:

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