If you're going to be covering up the hairline completely, a thicker polyurethane skin system with injected hair might be the most convenient option for you.
The hairline won't style backwards easily and the front edge can look kind of fake (especially when it starts wearing out), but it wil conform comfortably to your cranium, you wont feel the edges (if it's fitted and glued down properly and isn't overlapping your other hair, the crown growth pattern looks very natural, and the thicker base makes it less prone to tearing.
If you wanted extra hairline realism (especialy for pompadours and spiked or other back-combed styles), you'd probably want a thinner polyurethane skin unit with V-loops, or an ultra-fine swiss lace unit. However, these are both prone to tearing (usually if you try to pull it up after gluing it on, or while trying to remove tape), and the lace systems can get messy when adhesive seeps through into the hair, but these types of bases have more realistic hairlines. You should generally avoid using tape on these systems and use glue instead.
V-loops sometimes look less natural with dark hair, so some people will get a flesh-toned unit (or just in the front) and have it dyed to their hair color, which helps hide the "v" points in the base.
Other tricks you can use to get a more realistic hairline are a graduated hairline (thinner in the front, thicker further back), a staggered hairline (peaks and troughs of hair in the front), and (especially for lace systems), scalloped hairlines, underventing, and bleached knots.
There are also other kinds of bases you can get, like French lace or monofilament which are more durable but might not look or feel quite as realistic, and combo units that combine multiple types of bases (e.g. thick poly skin all over but Swiss lace in the front).
I used to wear a very nice-looking "Hollywood lace" front partial when I was younger, which I think was French lace with polyurethane-coated gauze around the edges and underventing in the front. The undervented hair hid the thick edge in the front so people couldn't see it, but you could feel it with your fingers and sometimes it dug into my scalp. Also, the adhesive from the tape I used to hold it on would seep off into the hair and make a mess after a couple of days, and when the bond started to loosen it would slide down my foreahead, causing my hairline to lower and a gap to appear across the middle of my scalp.
Now I only wear front-to-back ultra thin skin toppers. You can't feel the edges as long as they're glued down all the way (and not overlapping your other hair), and if the base is thin enough and cut properly (and not a different color from your scalp), you can't see the edge of the base.
