A good qquality hair system should not have a 'super detectable hairline'. In order to make the hairline look as undetectable as possible, I'd suggest the following:
1) If you like skin systems, get one that has lace (preferably thin Swiss) for the hairline. I found that skin on the hairline has a weird 'shine' to it that can look at a bit 'off' in certain lighting, plus I found that skin hairlines shed more quickly than lace, but I like skin for the rest of the system.
2) Consider the type of hairstyle you have. Do you have a totally exposed (i.e. combed/slicked back hair), semi-exposed or non-exposed hairline (fringe)? For me, I have longer hair systems (around 10 inches) and style it either combed back or in a central parting. With the parting, the hair on each side of the parting slightly covers the hairline, so it's not exposed.
3) Consider the type of glue or tape you're using for the hairline as this can make or break how realistic it looks. I've tried lots of types of adhesives and I now use Ghostbond Platinum for the hairline and Supertape for the back and sides of the system. Apply the glue in THIN layers to the hairline otherwise it will be more detectable. You might also like to try Walkertape Lace Front thins or 'no shine' tape for the hairline also. These looked ok, but I found them too fiddly to use.
4) The better the quality of hair system, (not necessarily more expensive) the better the hairline in my experience. The hair systems from my current supplier have excellent hairlines that can be totally exposed by combing the hair back and they still look natural. It might just be a case of you need to try different suppliers until you find one that has great quality, realistic looking hairlines.
5) Make sure you clean/redo the hairline whenever you're starting to get a bit of lift and build up (as otherwise the hairline starts to look less good). I typically redo my hairline every 5 to 8 days (I get 3 to 4 weeks hold easily with the Supertape for the rest of the system). If you need to clean the edges of the hairline without redoing the whole hairline, simply take a cotton bud (q tip), dip it in isopropyl then run it over the edge of the hairline (while holding the hairline in place with your fingers so you don't go too far with the q tip and cause the hairline to start to lift).
6) Make sure you've got the right shape/contour/width of hairline that looks best for you.
7) Make sure the hairline has a graduated density and (if using lace) that it has bleached knots.