Best bang for the buck, IMO, is the KB strobe and disc. It's very important that the strobe is battery operated and thus is not affected by drift of the frequency of your house AC. Furthermore, the KB disc is large in diameter relative to most, which makes for superior accuracy. Cost is $100, more than the cheapest and less than many. You have to go up to the Sutherland Timeline for superior accuracy, at ~$400. Cell phone apps are unreliable, according to scuttlebutt.