I'm gonna give another vote to Samurai Jack and Batman The Animated Series. Although if movies of a series are allowed, I'd say Batman: Mask of the Phantasm. Same aesthetic as the series, but on a higher budget and it shows. Also, it's just a plain good movie overall.
If movies in general are allowed, I'm also going to give a shout-out to Disney's Hercules. The style reminds of Greek vase paintings (sometimes overtly so) and gives it a really distinct look amidst the other Disney movies. As for the animation quality: it's part of the Disney Renaissance era for a reason.