I agree mostly with Incursio, although, people have different opinions on creepiness.
Anyway, I agree with the quality of work put into the Mega Gengar piece really sells a dark vibe. The attention to teeth? I love it, the detail of it and the smile. The values in the piece really helped it, too, which I think the Giratina piece could've played around with more.
With significantly more darker values and spots of light values, it can really sell for a creepy composition! Especially if you have the chance to show more of the shadows of Giratina and maybe only show the red drops and highlights? With eyes, it could add a little extra interaction. A cool reference I like shows examples of the same subject and different lighting situations:
I'd say check the post out for more help on mood and how lighting helps it if you think you'd benefit from it. But like lighting suggests, it can determine what mood the piece has. I'd suggest you get creepy and mysterious vibes from more negative space (the darker values) and less of the positive spaces. That's probably why we can get a cool and creepy vibe from the Gengar (personal opinion I don't wanna speak for anyone) because of, like Incursio said, the dynamic pose it has along with the composition AND the range of values really make it dark. It went beyond in dynamics (posing, lighting, composition ect) so I voted for Gengar because I appreciated the dynamics. Giratina is made well, it just could have been pushed a little further with values and a more determined lighting situation. ;u;
You both did an excellent job, especially concept wise and I wish the best of luck to both of you!