I've scraped tonnes of sites. I'm making these datasets for public use. I've been using GPL-v3 license for other software tasks but I don't know what license should I put for datasets. I prefer a strong copyleft license, would somebody recommend what license should I use?
The person(s) who went through a creative process to create the data are the ones that hold the copyrights to the data and they are the people that get to determine under what license they want to publish their work.
If the data in the data sets are just simple facts that are recorded without any creativity involved, like measurements of physical properties or sports results, then the data is not protected by copyrights at all and nobody can impose a license on that data.
So, depending on the type of data in your datasets, either it is not covered by copyrights and can't be licensed, or you need to ask the person who created the data for what license they prefer to have on the data. And that could very well be that you are not allowed to distribute the data at all.