Is devourer the best jungle item?

Depends how you play. If you really like farming in the jungle all day then it is the best. But if you want a jungler that ganks really well, or ganks really often, then it's one of the worst. Make sure you consider what you play to do and see what is the best in that particular case. There isn't simply a best.

I jungle a lot. For Jarvin, you will want the warrior with its armor pen and build tanky. For ww and fiora (I love jungle fiora!) devourer is great. Fiora will start off slow but gets a lot of good kills/assists and it stacks up the devourer. If you build fiora right as well, you can get a 2.5 attack speed with her e (100 damage stacked average at end of game) is extra 250 damage a hit and it applies on hit effects with ult like ww. For a champion like Diana you can go devourer or magi. I guess what I am saying is... High attack speed, get the Devourer... Lower attack speeds and tanky, anything else.