Worst: The super awesome Lastolite Brolly Grip Kit that can be used everywhere except indoors (light bouncing everywhere when using shoot-through umbrella) and outdoors (catches wind like an... umbrella). Replaced it with a no-name ripoff of the Lastolite Ezybox Speed Lite Plus which is much smaller but gives way more control and is more portable.
Best: Found a used-only-3-times-meticulously-repackaged-in-original-packaging Elinchrom RX One to-go softbox kit for almost half the price of a new kit. Only two downsides are the fans being a bit noisy and the radio transmitter using non-standard size button cell batteries lasting a bizarrely long time but having no warning for low battery level other than the flashes starting to misfire.
Second place for best is a tie between my light meter and a two-foot 5-in-one collapsible reflector that I didn't properly learn to use until a decade after buying it.