OpenNTPD's goals are to be "good enough" and provide "reasonable accuracy". On an OpenBSD laptop and several "play" VMs (running OpenBSD), it was indeed "good enough". For individual desktops or laptops and the random "standalone" machine, OpenNTPD is simpler and "just works" (I like that it can "verify" the time using HTTPS hosts of my choosing).
Nowadays, only my stratum 1 NTP servers still run the reference implementation. Everything else -- especially hosts which I may need to correlate events based on timestamps -- runs chrony.
A comparison of the three implementations [0] is available on chrony's website. From a quick glance, I don't see anything blatantly incorrect or "biased. The comparison was discussed here on HN ~18 months ago [1].
Basically, if accuracy to the second is good enough, OpenNTPD is fine. If you want more precision than that, go with chrony. It'll be MUCH more accurate and it really isn't any "harder" than OpenNTPD. You'll probably want to stick with ntpd if you're using reference clocks, although chrony supports a subset of them. If you're a nerd that wants the absolutely most accurate time you can get, Google "PTP 1588" as well.
