Fluorocarbon is definitely the best choice for level lines, but not all fluorocarbon lines are the same. Personally, I really value having a hi-vis line. When fishing wet flies or nymphs, you can easily see little twitches or hesitations that may be the only indication of a subtle strike, and when fishing dries it will be much easier to follow your cast and find you fly. I am convinced you will catch more fish if you use a line you can easily see. The only problem is that in the US, the biggest selling point for fluorocarbon is that it is harder for fish to see, so almost no one markets a hi-vis fluorocarbon (although if you do a google search for hi-vis fluorocarbon tenkara line you can find it). I've tried to use clear fluorocarbon,which was impossible to see. I've also tried Stren's fluorescent blue and Berkley's transition, and although they are easier to see in full sunshine, I still find them hard to see in deep shade or late evening. If you get something like a fluorescent orange, it will be easy for you to see, and the fish don't seem to mind it.