Fishing line is the only connection between you and your next trophy fish. In my humble opinion, it is probably the most important part of your arsenal. Skimping on line is definitely not worth the price of a lost trophy.
I prefer superbraids to any other line such as monofilament or fluorocarbon, as you can't beat the knot tying capability of a good superbraid. In addition, their ultra thin diameter allows for loading up on lighter gear without giving up line strength.
To be more specific, I use 14 LB Berkeley Fireline for most fishing situations, it was a nice 6 lb test diameter. It works great on light to medium action rods and reels, and the knots I've tied have yet to fail. Aside from being ultra sensitive so you can feel the bottom when drop shotting or jigging, you can also get really good hooksets with a quick wrist flick as opposed to having to jump halfway across the boat as you see some TV guys do. I use it to cast or troll for bass, pike and walleye.
The exception to the rule for me is when it comes to carp fishing. As I fish a particularily rocky area of the St Lawrence River for them, even 30 LB Fireline won't do when it comes to these powerful fish running the line against sharp / zebra mussel covered rocks. In my case, after trying many line, the one I'm the happiest with is 50 LB Power Pro that comes in a 12 lb diameter. Besides offering great casting and easy knot tying for a line of that strength, the spectra braid used in it's production is simply amazing when dragged against rocks.
My newest set up that I'm waiting to try is a 20 foot shock leader of 50 LB Power Pro, tied to my regular 14 LB Fireline using the back to back uni knot, set up on a medium/light action rod and reel combo.
Will post results to this test when I manage to hookup to some carp on that setup.