I bought some frozen, boneless chicken breasts that I wanted to cook tonight. Should I leave in the package and immerse it in hot water, or should I thaw it in the microwave (in or out of the package)? 
Are you cutting up the breasts for your application, or keeping them whole? I ask because boneless chicken breasts (as well as just about everything else) are easier to slice cleanly if they're a bit (but not completely) frozen.   How much time do you have between "I have hard frozen chicken breasts" to "I want to start cooking"? If it's morning where you are and you want to cook tonight, just throw them (still in the package) into the fridge. If they're not completely defrosted (and you want to keep them whole) when you're ready to cook, just run under cold water (still in the package, just for convenience sake) for a bit. It won't take long, once you can easily separate individual breasts, just a couple of more minutes separate and out of the package under cold water will give you a product ready to go.  Whatever you are doing, there is no harm in the very center of the breasts still having a bit of a frozen chill when you start. Boneless skinless chicken breasts are notorious for drying out (becoming overcooked) before the surface reaches a nice brown. A slight frozen chill at the center is a good thing. 