If you're talking Internationals (6/21-6/23) in Columbus, you probably should wait for the next regional or two in order to see what's good... in particular for example if Reshizard dethrones Pikazek and Zapdos, or if Baby Blacephalon comes out, or even something different. If I had to guess now, I'd play one of those first three - but there's a _lot_ of time between now and then. Play all three with proxies or whatever you do for testing, and see what's what.
If you're looking to decide now and want the safest pick... it just depends on what you're comfortable with. Aggressive play? PikaZek or Reshizard both fit that. Consistent play? Zapdos.