Really hard to say man-- remember, One Piece is a Shonen manga but it's a little kid's anime (even moreso than Naruto and what not-- it appeals to a wider audience, and therefore, they have to make the anime version even more tame). I personally dislike arlong's voice a lot... so I'd say at some point after that arc. Dunno, starting at Alabastia would probably be fairly safe, but honestly I didn't like the animation until more around the Shabondy arc. Honestly, even Ernies Lobby arc has pretty crap animation overall (and the anime never reaches "great" quality, but you can't expect a lot from any long shonen series not named Full Metal Alchemist). This is not to say I like FMA better than One Piece, but just comparing the anime, FMA is definitely superior. One Piece is one of those anime that still makes you lament about how much more epic the manga version was most of the time.