So long as we're talking about '2D animation' from well funded projects (usually cinema / ova, but occasional series), there hasn't been any backstepping since CGI painting and rendering started to be commonly used, and every new project is visually more exciting than the last.
Talking about the typical TV series, I'm tempted to say the retro-look of 80s animation is most appealing, but I think it's more likely that there was simply more of a distinctive look. The typical noughties series looks sterile, CGI looking cheap and not mastered in the average studio (though I wonder whether it has at least enabled certain projects to be commissioned cheaply when they would have never stood a chance). I'll go with the 90s because I think the industry's heart was in it. And by heart, I probably mean wallets. They animated things back then.
As an aside, the animation of robots: difficult in the 80s and usually pretty flawed, modern CGI trumps it but it nearly always looks out of place. Recent mecha series like Gundam 00 and Macross Frontier are getting it right, but I think hand-drawn animation had its moments before it was largely done away with. I've always felt that Turn-A Gundam looks brilliant for one.