Closer:

There really is no one line that will do everything well, but most quality lines (expect to pay $60 or better) will handle most situations at least marginally. As others are saying, if you want to be dialed, it really depends on what type of fishing you plan to do most.

You'll get good advice here, but ultimately, you will be the one to decide what line is best for the fishing you do. Everything about the way you fish, be it the water you fish, the techniques you use, the types of flies you fish, your preferred rod action type, or your casting stroke, will factor into what lines work best for you.

All that said, for all-around use, I have personally enjoyed the Scientific Anglers steelhead taper line I currently have on my 6-weight quite a bit. It's not great for nymphing (a lot of the weight is in the middle and back of the line, so shorter casts are somewhat difficult), but for greased line summer run fishing, it's hard to beat. Makes it easy to keep a lot of line in the air and mend it on the water.

Good luck, and have fun learning.