It's a tough decision between Volga, Midna, and Fierce Deity/Young Link.
I love playing as Volga simply because he's awesome and a giant freaking dragon.
I love playing as Midna because, well, She's awesome-looking, and because I love blasting down tons of enemies with the ancient/sky cannon.
I don't really care for playing as Young Link himself, but I FREAKING LOVE the Fierce Deity mode. I know how to play him a certain way, (one in which you can constantly be Fierce Deity), so, I can use him tremendously and awesomely.
By the way, I refer to "twili midna" as just Midna, or True Midna, simply because that's ACTUALLY her, and twili is redundant because we already are aware that is her 'race'- if you will. It's just redundant. -and annoying to me.
I call the other form, Imp-Midna to acknowledge the fact that isn't really 'Midna', since that's just a curse of sorts.
In short- it's not who whe really is, it's not her true form, so I think this form should have an additional name indicating that it ISN'T her true form.