You dont say what type of fly line you are meaning, e.g. floating, intermediate, fast sink, etc
If it is a floating line, then the Rio Gold is hard to beat. Expensive at around £50 but woth every penny. For a 'middle of the road' price, the Snowbee XS and Greys Platinum at around £35-40 are excellent lines. I ahev used the Snowbee floater for many years and found them to be excellent
For an intermediate, my choice would be the Greys Platinum clear intermediate or the Cortland 444 Classic Blue. Remember with a clear intermediate you will get more memory in the line as the core is made from mono and not braid. Prices are £35-£40
Fast sink line choice for me would be the Rio Deep7, quite simply superb. Price is around £45
Fir different rate sinker e.g. DI3, DI5, etc, the Airflo lines are very good, just dont toucvh their floating lines!! Also very good are the Scierra HMT lines. Prices are again around the £35 - £40 mark
You can get cheaper lines but with fly lines you do, quite simply, get what you pay for