Advice: Defiant, unbearably stubborn 2 1/2 year old?

I'm sure my daughter behaves like the typical 2 1/2 year old: Constantly testing the rules, stubborn, daily meltdowns, "my way or the highway" attitude, but a lot of the times I feel like she's more defiant than the average 2 year old. I swear she's a 16 year old in a two year old body, she's such a drama queen most of the time. Apparently what I'm doing to correct her behavior isn't working and I'd like another perspective. Sometimes she does well and listens to me, but most of the time it's not nearly as easy. It's a constant battle and I try hard to avoid power struggles because I know they lead to no where. If I tell her to stop doing something she either pretends like she didn't hear me or (even more frustrating) she'll look me dead in the eye and keep doing it. I'll ask her again more firmly and then when I go to get up to physically correct her she drops what she's doing and run away laughing. Time-out doesn't seem to work either because she'll just sit in the corner, throw herself at the wall and scream for 5 minutes (and then she's back to doing that I originally put her in time-out for like 10 minutes later). Tried spanking...didn't phase her. Tried rewarding her for good behavior, but that didn't work either. So now I'm kinda stuck. Lately I've been working on controlling my temper because I noticed that I do raise my voice a lot (which obviously isn't helping) and I don't wanna be one of those moms who's constantly screaming at her kid. Advice?