I don't fly-fish, so my advice is for regular rod & reel. I've got hundreds of trout on 6# mono, caught my best trout ever this spring on 4#, but I've been really impressed w/ the flourocarbon Berkley Vanish I bought for my bass rod. So, I bought some 4# fluoro to put on my ultra-light spinning reel. The stuff is strong, and really disappears in the water. Yeah, it's pricey, but I think it will last longer than the mono I've used for years. Also, it has hardly any memory, so w/light lures, like I use for trout, that's a plus. Just remember, trout have good vision, and spook easily, so heavier test line can sometimes be unsuccessful. Having said that, I watched my best friend pull in several rainbows and browns w/ my Zebco 33 spooled w/ 10#.

Depends on what kind of fishin you are doin. I use 6lb floro on my ultra light. 10 lb spiderwire on my trolling setup with a floro leader. 6lb mono on my spinnig rod. Always buy premium line! If you go and get it towards the end of the season you can find it on clearance at wal mart. Just got a 250 yd spool vanish floro for 2 bucks.

I like berkly vanish and i use it in 4lb test