You never want to wake up in the middle of a Deep Sleep or REM sleep cycle. The best time to wake up is when you're in between cycles. Which is what the "Smart Alarms" are supposed to do, they wait until you come out of a sleep cycle to wake you up.

The Zeo is the most accurate (since it picks up brainwaves), however it's also slightly uncomfortable and buggy. I personally use the Jawbone UP which works good enough. All of the apps that say they do that aren't the most accurate since they rely on the movement of your mattress. However I know some people who swear by them.