Which is the best site to stream One Piece in the highest quality for free? The first episodes are so pixelated in the sites that I use

That's just because the early stuff was 480i. There are some upscaled versions available, but I'm not sure if anyone streams it.

Might be worth asking over on animepiracy@lemmy.ml, but like the other one said: it's old. So, you may have to deal with it (unless your source is so bad that it's worse).