TOPPIK with the spray applicator. This is the best thing by far, buy it and try it out. Make sure you spray into your hairline but not directly on your scalp, also wipe your forhead with a tissue after applying it and gently wipe the top of your hair as well to remove excess.
Couvre is actually good for the hairline as well, however, the applicator it comes with does not work for me, I had to do it with plastic gloves. This is one of the first concealers I started using, it worked fine but I stopped using it a long time ago because frankly it was a bit messy, I did not like the smell at all and I really couldnt scratch my head or my fingernails would turn black. Also Toppik was just so much quicker/easier to do when rushing out the door in the morning. Not sure how Couvre would hold up if you're going swimming, I've never used it for the beach/pool. I have not been to the beach/pool in 4 years since I first started to notice a little thinning and began using concealer. Sort of miss it but of well..
Dermatch is good, it has no horrendous smell like Couvre, but personally I just don't have the skills to master a 100% realistic hairline concealer with Dermatch. Also, it turned my sink into a black mess. Many people here swear by it however and it definitely would be something to use if your going to get caught on a heavy rainy day(although a light amount of toppik does hold up well in light/medium rain). Dermatch does last a very long time as well.
If they ever created a product with the cream texture of Couvre but the light almost unscented fragrance of Dermatch then it would really be a hit.
I've never tried any of the other sprays such as Prothik ect.. but they seem easier to use than the ones listed above for the crown and vertex. Perhaps the outcome isn't as good and that's why people don't mention it? I don't know. For the hairline I don't see how it would work though.
All concealers require an "art" to applying and usually require a set of tools or other strategies to achieve a real and fantastic natural effect. Not everything is meant for everyone, it all depends on your hairstyle, hair color, hair texture, how innovative you are and how much time you want to spend each morning ect.. I have never been able to use any of these and be happy with the results by just following the directions "as is". If you're persistent and spend the time to try out different ideas and strategies with these concealers you will surely find a routine that works the best for you with results that you'll love. Just make sure you never get to a point where you really start to slather this stuff on and it becomes obvious.