Ok, since no one wants to talk about Marth or G&W... >_>
Marth is a beast. He (or she? lol) pretty much has superior range, damage-racking ability, and killing potential. Dtilt pokes are detrimental to his spacing game, but I do not believe it does anything to Jiggs; we'll be in the air anyways. He (well, I'm not going to say it completely shuts down our aerial approach, but it's close) has jabs and retreating fairs to make our air approach difficult. His OoS game is amazing. A Dolphin Slash or DB OoS is kind of annoying to fight against. He can kill us pretty early if his upsmash or uptilt is kept fresh. Actually, a tippered, fresh ANYTHING pretty much screws us over at high-percents. His off-stage presence is also above-average, utilizing his amazing edge-guarding with his many aerials. Often, if you're below the stage he'll ledge hop below and attempt to spike you. He can also spike even if you're not below the stage but when you're trying to recover linearly from the ground off-stage. A spike, even at a low-percent, is VERY bad as it could mean death if Jigglypuff runs out of her jumps.
But I think I over-estimate Marth. Sure, he outranges us... But to tell you the truth, Marth cannot kill us easily if you are aware of his spacing game. You basically need to outspace HIM, which is pretty hard considering Marth's tools compared to our own. Empty SHs and retreating aerials are your friend. I suggest not staying too close on the groundto Marth all the time, as he could just Dolphin Slash you away. Utilize Jigglypuff's superior grab game, imo. Puff's throws lead into a lot of aerial abuse. If anything, try to be below Marth when in the air. What's Marth going to do, dair you and risk whiffing it AND being subject to HUGE ending lag? Uair is a good juggle tactic here.
It's a battle of wits and spacing here. Marth will probably win the spacing game though. 40-60 Marth.
Idk what to say about G&W, sorry -_-