My personal favorites are Peach, Jiggs, Captain Falcon and Fox.
Peach can hold stocks like nobody's business, and you can down smash/grab combo if you get the spacing down right. Turnips for a projectile game, and that handy Up+B that she can use to save you if you make it close to the ledge make her a good pick to support Marth.
Jiggs opens up possibilities for rest/grab combos, as well as the puff punch to save you when you're dying off the side of the stage. The only real problem with Marth/Jiggs is that they have no projectiles, so anyone who has them will give you problems if they camp.
Captain falcon is ridiculously good in teams. He gets to fly all over the stage popping out great aerial moves, and its hard as hell to edgeguard him when there's a Marth watching his back at the ledge. This pairing puts Marth in more of a support position, but they both have good grabs that combo easily into each other's moves. A lack of projectiles here is slightly made up for by the incredible speed of Capn' and range of Marth.
Fox makes for a good pairing with Marth IMHO because he can do so many things to a grabbed opponent. Lasers, up-smashes, d-air to shines, all of these are things your competition won't be happy with. I agree with Marth_marth that a defensive Fox works better with Marth, having Marth get the grabs and edgeguard, while Fox racks up damage and gets in finishing blows.
