League of Legends (LoL) Question: Which Rune is better one Zoe? Electrocution or Dark Harvest?

Definitely Electrocute. It is much more reliable and effective in terms of total damage dealt. Dark Harvest might be fun and all, but Zoe isn't able to proc it's effect enough times in order for it to be better then Electrocute. Electrocute is a rune you can use at any time, no matter how much health does your enemy currently have. In case of Dark Harvest, Zoe can proc it only on targets below 50% HP, which is not an easy task by any means.

Electrocute is the rune, that Zoe has highest winrate and highest playrate on. It is powerful, easy to proc, reliable and definitely better then Dark Harvest. The only case I would recommend Dark Harvest in, is in faster and deadlier game modes like ARAM or ARURF. There, it is much easier to get your opponent below 50% HP, and Dark Harvest is definitely just as good if not better then Electrocute in those game modes.

Additionally it all depends on your Elo and how people play into you and also how long games last.

Electrocute is as Tauricus mentioned the better and more reliable choice throughly early-mid.
Dark Harvest is the best late game rune that lasts upwards to 40-50 minutes, sacrificing early.

You need above 25 stacks to out-scale Electrocute, if you can't manage this or your games are lasting 15 - 20mins on average it's simply better to stick with Electrocute.