I use 14 lb Trilene for all reel fishing. Bait casting or spin casting. Good all around strength without being stiff.
I never look at yardage when I'm filling a reel. Once I get what I feel is a good base, I'll add 30 to 50 yards of "working" line. This allows for removing abraded line as it developes working through eyes, fishes mouths and structure like rocks and trees.

Bubba

I think cover is the defining factor, along with water clarity. I caught an eight pounder on six pound test test the other day. If you have your drag set right, and you can keep them from wrapping cover in a fight, it's not really a problem. On the other hand, if you're really in the thick stuff, heavier line is in order. Length will be dictated by the reel. Too little and you will not cast as far, too much and tangles may become a problem.

@ethan3...I agree with you as to the first part of your response, as to setting drag, I have probably been fishing since you were in preschool based on your picture. Apparently we have a different definition of what heavy cover is. Whether it breaks or they spit it, you don't catch them if they are way down in the thick stuff and you are using light line. You just can't put enough pressure on them to keep them up out of the weeds. In open water, yes you can catch 3lbers on 6lb, you can catch them on 4lb if you really want a fight. That is a totally different situation than fishing heavy cover.

I like 12 lb. test for my bass fishing. I fish in clear water and the slightly smaller diameter helps, I think. I'd probably go a little heavier if the water had more color---you never know when that big boy is going to strike.

Really depends on what it is for. I have about 6 rods set-up for different types of fishing. My flipping stick has 20 lb. braid. My spincast that I fish down to 25 feet has 8 lb mono. A good all-round line would be 14 lb mono or 12 lb braid.

For a baitcasting reel, I would go with 14lb mono as well. If this is your only bass fishing rod, then that will do most stuff for you. Spool it all the way to an eighth of an inch from the end of the spool.
For a spinning reel, i would go with 8 or 10lb test mono. It will have less tangle on your spinning setup with a little smaller line.

Like others have said, it really depends on the bait your are using, the water color, and clarity. I have a spinning rod with 8lb florocarbon for finess, a med action casting with 12 florocarbon for cranking, another med with 14 mono for top water, and a med heavy with 14 flore for spinnerbaits and bigger swimbaits, and some soft plastic, and a med heavy with 17 floro for heavy cover stuff. Fill the spool up it makes the reel cast better. And for Braid if you must use it I recomend a 14 floro leader.

I love to sit and read all the responses about line. Each and everyone has their own ideas about line and what size it should be. That is why so many different line types and sizes exist in the first place. Some people find great pleasure in fighting a fish for a long time after they hook it and seeing just how small of tackle they can get by with. Others find all the pleasure in just setting the hook. Then cranking the fish in quickly so that they can get the hook back into the water and continue fishing. The reels that get used to catch these fish also play into the formula. They start with huge electric reels for deep sea fishing and go all the way down to fly reels for fly fishing.

Fishcatcher96, you did not tell us what size or type of reel the line was going on. Please tell us more about the reel and where you plan to fish.