I wouldn't say that Jiggs is the best at teams, but she's deffinately up there. I think it goes more like this.
1. Shiek-she can serve both as a stock tank and as an attacker, so she's very versitile in teams. She has no trouble handling herself and can even interupt your opponent's combos with a quick needle. She can use her needles to either save you or keep your opponent from edgeguarding you. She's great at getting grabs to help you with team combos, too. She also has an easy time finishing opponents with her "anything to fair" combo.
2. Fox-shinespikes help get an early lead and he's pretty good at taking care of himself. He lives a little longer in teams because of his partner's ability to save him from most on the stage edgeguarding. He's pretty good at keeping an oponent busy so you can do your thing un-interupted. He can't really save you, but what he can do is kill the person edgeguarding you, or at least destract them enough for you to make it back. His up smash and uair are more spamable in teams, because it's harder to keep track of what's going on.
3. Marth- his long range allows him to pressure multiple opponents at the same time, and he can easily help you regain your composure. He actually lives longer than most people think because his recovery actually has a very good range as long as it's not interupted. Most of the time your partner will be able to defend the ledge for you, so you can be the stock tank as well as the team's offense. He doesn't have quick kill moves like Fox or Shiek, but he has the best edgeguarding in the game and can kill people long before kill moves would even be effective. Saving your partner isn't that easy, but Marth can use his side B to reel his partner towards the stage. He can interupt your opponent's combos with a simple fair and go straight back to what he was doing. His throws are great for setting up team combos and for just keeping opponent's busy in general.
4. Jigglypuff-I'm not quite sure if she should go above Peach, but Jigglypuff is significantly better in teams than in singles. Rests are much better in teams because you can use it more often and with less punishment. Jigglypuff's edgeguarding is in the same league as Marth's and her bair is a nice spammable move that sets up for edgeguarding ,too. She's great at saving her partner because she can go pretty far off the stage to do that. My only real complaint is that she has a hard time keeping up with the action, and her being light certainly doesn't help. If she gets knocked off the stage then it will be quite a while before she makes it back and that will happen more and more as her percent rises.
More Later.....