Plank is a good choice for those looking for something simple that just does its job well. There is no UI for options built in to the dock as to change anything those options will have to be accessed through a text file.
Other file explorers and terminals (those not found in the default elementary OS) do not have any right click menus for the dock meaning it is impossible to launch multiple windows for certain file explorers or terminals.
When configured to only show apps from the current workspace, cycling through windows of an app open on multiple workspaces will switch workspace focus, update the dock, and often leave you to cycle another app.
An additional third party app has been developed that allows theming the dock without the use of any extra resources. This is great for those that are not using elementary OS and the elementary tweaks extra.
There's no padding setting for the dock. So autohide gets triggered easily by mistake. You can configure a delay instead or add padding in the theme (though it will get applied to the items and makes the tooltip position further away too).
Docky and Plank is maintained by the same person. Plank is still getting occasional work, but Docky hasn't since 2015. Plank uses GTK3, while Docky still uses GTK2.
Docky is quite beautiful, and extremely customizable. It also has many features you would expect from a robust dock. It is the nearest thing to Apple's Dock on macOS.
Although the application itself is light fast and efficient, it depends on Mono, which is a large library (~250mb). If you have enough storage or already have/use Mono this isn't a problem.
gconf editor is required in order to remove the Docky icon from the dock, which seems unnecessary as the setting exists, it should be in the app settings instead of hidden away in gconf.
Docky is notorious for being quite unstable on older hardware that cannot use newer drivers for their GPU or newer kernels. Most often people have trouble keeping docky open, as it will shut itself down often, and won't start on boot.
DockbarX has an available plugin called "DockBarX Xfce4 Panel plugin" that allows DockbarX to blend in seamlessly with the XFCE4 panel, which is great for XFCE users due to the limited options of the default panel.
Cairo dock proposes a feature rich tool, tweakable to any degree. The configuration menus are at times inappropriate, shallow and uninteresting, complex to navigate, and prefab-centric. Rarely offering a satisfying preset, and always unsatisfying to tweak.