"I don't like commenting on things about anime series that people are currently watching which I have seen, and will try to refrain from doing so in the future, but I like how the art style changes over time."
In a comment, John Lawler wrote:
They're both fine, and they'd be fine in the opposite order, too. Whether to use which or that is strictly a speaker's choice, depending on how they want it to sound. But another tendency is not to stack relative clauses up one after another like this; better to reduce the more restrictive one and put it right after series and leave the other at the end: series I have seen that people are currently watching is easier to read and puts the ideas in order.