I was asked by a fellow angler just what kind of line I use on my reels, the only answer that made sense was "as big as your reel can handle!".  I'm not going to get into a big wee wee match over what's best for everyone because there are to many different scenarios for different lines to talk about everything, so I'll only mention what works for me.  My big single action mooching reel is always dressed up with 25lb test mono, flavor is up to you but I like the Maxima Ultra Green and on one reel I also am trying Berkley Big Game, and it seems to be working well!  On my level winds I have one with 20lb Maxima while the other sports 30lb braided Spider Wire, only because I wanted to try some low stretch line for those long line times when the fish just don't hook up well.  Just make sure if you use whatever type of low stretch line that you have a rod that is somewhat supple, as it will be taking up all the pounding the fish gives as the line won't be supplying any shock absorbing help!   This stuff works real well for that and if your concerned about the fish complaining that they can see the braided line then dress the tip with a chunk of fluorocarbon and that'll stop their whining!  I know, there's those out there that say the line is too heavy and it'll spook the fish and they always use 8-10 lb max and will catch more fish than me!!  Whatever!!  If you want to try and bring up a 10lb laker on 8 lb test line, by the time you get it to the surface the thing will probably not survive the release.  Sure, it can be done, but I'd rather have a little more strength to my line and get the fish in quicker and release it quicker, to each their own.  As well, when you use a downrigger as much as I do, that lighter line will just not stand up to the abrasion caused by that clip.  When I first started out on this "adventure" I actually had line break on me right where I always clipped to, and that was 20 lb test line!  I learned my lesson and now even with the heavier line I change the "clip spot" as much as I can.  There is no doubt that there are situations where that lighter line will catch more fish, such as fly fishing, where the fish actually has more time to view the meal.  However, when your trolling deeper water and doing speeds of 2-4 mph, I don't think the line is going to have to much play in if the fish strikes, especially instinctive strikes, they're just gonna pound the lure!  Hey, if you are uneasy with the heavy line to what the hook is attached to then do as I did and go to a bit lighter leader, but just make sure its compatible with the rest of the line, in other words don't be hooking 6lb test to the 25lb test.  Oh, one other line set up I do have on board is another single action reel with some 50lb braided line, you never know when you'll want to throw on some jigging stuff for cod or lakers, and that's where the low stretch really works well on the hook ups!!  Trial and error and it all depends on what kind of fishing you want to do.  If your a troller then a bit bigger line, if your a spin caster then a bit lighter line (my spin casters have 10 and 12 lb test on them) and a fly fisher, well, go with your 2lb test tippet, I'll keep my dressed with 5lb just cause!!  One point I will make about lures though, if you notice those Apex's your buying have different line strengths, some have 8 and some have 30lb!!  Over the years I've changed up the lighter weight ones to heavier material and they still perform and catch as many fish as the light stuff!