It depends what you want it for, how many bikes you want to use it on, what pedals you use, and other factors
Best thing is to read DC Rainmaker's power meter summary and get the strengths and limitations of each.
For me, Stages is a good option. I have two of them, which get shared between three bikes. Ideally I'd buy a third one but it's only five minutes to swap them over. Garmin pedals wouldn't work as I don't use that pedal / cleat system.
4iiii is a slightly cheaper alternative to Stages, and there are other cheaper options. I've also got a Powertap Powercal (heart rate strap that gives power readings) which would be fine for much of my usage which is about pacing the start of longer rides. I got my first Stages before I got the PowerCal, otherwise I might not have bothered with Stages at all. But it would be no good for short interval training.