I use scanlines and I think it does a great job in improving the image. Specially if you're using AdvanceMAME on a CRT computer monitor. It knocks off some of the sharp edges of the pixels, without the excessive blur of pure hardware stretch.
I'm not claiming it successfully simulates the look of an arcade monitor. I'm just saying it makes the image more pleasant (to me, at least).
The reason I mentioned AdvanceMAME is that it can double the original game resolution and use every other line as a scanline. On a CRT monitor, that's probably the nearest you can get to an arcade experience.
The problem with LCD monitors is that they have a fix resolution and a different aspect ratio, so it's impossible to get the exact double resolution on most of the games. You usually end up with a kind of wave effect vertically. However, I do use scanlines on my LCD, I just raise it up to 90% so that the effect is more subtle.
At the end of the day, the best filter is the one that works for you. Just experiment with all the possibilities. That's not an exact science

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