that's quite a tricky topic, personally i like me a nice pdi 6.01 stainless, been getting good results although you do have to be a bit more religious about keeping it clean.
i've heard good things about the orga barrels, not used them myself but a mate has one and is quite pleased with it.
have a zci in the e&l and it seems pretty decent, although currently it's accuracy is more limited by running open bolt hpa than the hop/barrel setup.