The hardest part of being a dad (or mom)

As I write this Anya is screaming and carrying on in her room. She's approx 3 months into her terrible twos.
She's really starting to work at manipulating us and it's heart breaking. I feel like it's become my job to make behavioral mountains out of mole hills. Giving time outs, and raising my voice at the slightest offense. I've even spanked her once or twice (though very softly).
But today she did something that terrified me. We were at the Ace just goofing around when she decided she didn't want to listen to me anymore. She lay down on the floor. There was no screaming no kicking or yelling. She just looked up at me with a "what are you gonna do about it?" look.
I tried to make her stand in a corner and look at a wall (the "public time-out") but she refused to stand. So I threatened to spank her, and the expression didn't changed one bit.
So I gave her three light taps on the butt and she didn't even blink. I felt a serious "is that all you got old man?" vibe from her.
I suddenly had a very sad vision of what the next 16 years of my life could be like: An endless series of ridiculous, meaningless power struggles. I want Anya to devote her energies and intelligence to something other than trying to get our goat. I want to be able to go out and play and enjoy my kid without it having to turn into an ever-escalating exercise in discipline.
I'm pretty disappointed that she's behaving like all other kids in this regard.