ITT we design Z moves so they are an interesting mechanic rather than a shitty boring gamefreak gimmick. As in, don't just make them do a bunch of damage. Since they are limited to once per battle, you can really push the limits.

I'll start

Spark Surfer - Alolan Raichu exclusive
>120 power
>100 accuracy
>special
>If the user is faster than the target, paralyze the target.
>if the user is slower or equal in speed, increase the user's speed stat one stage.