Personally I find the A/B tool the best. Thumbnails are super important. But also the keyword research tool and the seo studio. If your videos aren't searchable, and have short generic tags, they aren't going to go far. The tag tools help your tags go from red, badly optimised tags, to orange and green well optimised tags. It won't do everything automatically for you, but it will give you an idea if your variations that you think of, are better or worse than what you already have.