That depends on what your definition of "best" is? If you mean cheapest, then you should Google the model you want to buy, find the cheapest vendor and pull the trigger. Yes, it is that simple. If you are looking for something else, like customer service or quick shipping or return policies, well then you have to be specific with us. Remember, the "vendor" doesn't build the thing, they just act as a middle man and make money off you. I would suggest checking return policies in case you have a defective unit. Good luck.
All of them are competitively priced. I bought mine from Maryland speed and it took about 6 days to arrive by mail (free shipping) I got the tuner+thermostat package...although i wished i'd chosen the more expensive thermostat, instead of the one they throw in if you don't. I also got the USB cable as part of the package, not that i needed to update it or anything but better to have all the parts when i mayneed it for later.
Its pretty simple to install too, only takes about 15 minutes to pull the original tune off and put the new tune in. I didn't set any parameters yet, just loaded the basic 91 octane stock tune. I haven't put the thermostat in yet, and probably won't until the weather warms up.