What ability you should use really depends on which setup you're planning on giving Smeargle. I myself run a Spore/Transform setup with Entrainment and Aqua Ring as the other two moves, Timid nature, stat experience investment primarily in HP and Speed (252 each), and Own Tempo is generally the better ability for it (Moody has too much of a chance of getting Speed as its down stat, and you want to go as early as possible on your Transform turn)
I am a die hard Pokemon fan to the most, including the traditional hate of Yu-Gi-Oh.
For PWT it depends on which type of battle you'll be doing. Smeargle has a lot of tricks he can pull off with partners. If you're going solo you should be fine using Focus Sash, Sub, and Spore to set up enough Moody boosts to sweep.