Very roughly in ascending order of the damages they work at (to the extent that they can be ordered like this at all):
• Edgeguarding (like you would do at pretty much any damage; it's always an option to consider).
• Raw forward or down smash (unsafe, kills early).
• Tech chase kills (probably off a throw; kill with up-B, forward smash, down smash, or maybe Dancing Blade).
• Onstage dair to up-B or forward smash, or to nair on floaties or at higher damages (more viable in PAL than NTSC since dair doesn't send the opponent as far away and also consistently/conveniently sets up for tipper forward smashes; kills early).
• Dancing Blade 3 or 4 (either catching them acting out of shield, shield poking, or hitting with only one hit in neutral; otherwise it's going to be DIed out of).
• Back throw to up-B or forward smash DI trap (hits on DI in only; works well by the ledge). Back throw can also be used to make the opponent land on a platform when up throw will no longer do so (where it will set up for forward smash, most significantly). Frame data for back throw to up-B/forward smash against Fox:
http://smashboards.com/threads/118998/page-394#post-19715256
• Raw up tilt (unsafe in neutral against grounded opponents, good and relatively safe at catching opponents coming down from the air; kills pretty early).
• Raw forward tilt (tipper is surprisingly strong and safe; good at catching opponents trapped by the ledge).
• Side-B to up tilt (not always guaranteed, if ever; possible to DI away or jump/airdodge out).
• Nair (won't kill directly outside of Yoshi's Story; Marth's strongest aerial at relevant damages that also sends away).
• Up throw (guaranteed kill but only works at very high damages; above 200% or so except for cases like Jigglypuff on Yoshi's Story).
Marth's zoning still works at high damages even if his combos don't, so don't be afraid to continue playing safe and going for positional advantage/damage until you have access to more reliable kill options again (rather than resorting to fishing for raw forward smashes once the opponent hits 100%). If you can keep the opponent out safely the whole time, then you don't really need an immediate kill (at least not necessarily urgently enough for it to be worth the risk). Get the opponent in the corner, or offstage, or above you; work from there, while the opponent can't take advantage of you not killing them.