if you do a little search over this group, you will find plenty of answers for same question.

i like shutterstock's suggestion tool, i use it to quickly get to 50 keywords and make sure i include a few important ones of my own, then copy over to other agencies

There were only ever two tools, as far as I know, that used actual data on what buyers searched for when buying. They have now both been retired.

All the rest are much of a muchness (based on what other people have used) so whatever you find easier. SS's one is fine.

I only use Shutterstock keyword tool. It's helpful. Although they made a couple passive aggressive changes recently.
1. They reverse the keywords order when you copy them, so that the most relevant ones would be in the end. It doesn't matter for Shutterstock, but it matters for some other stocks.
2. They now give you the worst, ugliest pictures from the very bottom. Obviously they were created by beginner stockers. And we get the keywords from them.
Also, I don't like training Shutterstock's search engine by inputing keywords, picking relevant images and picking more relevant keywords.