I've tried sooooo many sleep apps, but the one I always come back to is AutoSleep. The developer recently released AutoWake which is the perfect companion alarm app for me - it wakes me during light sleep in the 30mins before my alarm and you can set auto-alarms if needed. Really easy to use.
I like Pillow second-best and was excited when they introduced automatic tracking, but unfortunately it can't be used with the alarm set (the alarm also supports light sleep phase activation, but this never worked for me). So I don't use that at all any more. SleepWatch is 3rd choice for me - looks very pretty and some interesting metrics, but just doesn't seem to grab me.
I do miss the Fitbit sleep tracking - I particularly loved the multiple rows so you could see several night's sleep at a glance. The nearest app that does that is Sleep++, but it's the crudest interface and the peaks and troughs that are so clear on Fitbit are more blurred together. Another is Sleep Tracker, but I found it hit and miss at detecting sleep.
There's also Sleep Pulse 3 - no auto tracking and the interface looked prettier than it was useful. I use their companion PowerNap app though and that works nicely.