Fox, Marth, Sheik, Falco IMO.
In a 1 vs 1 falco has several advantages. No question if you get him in a position where he must firefox you can say good-bye and those shield spikes are really fulfilling. But at the same time falco can just damage fox up so bad and his aerial attacks, splits, and those super drumstick sidekicks are absolutely brutal when he catches you napping. His short hop aerials can really get on my nerves. I play fox and my friend plays falco so this matchup happens all the time. Basically it comes down to how well I am able to use my shield and predict his attacks. In free for all with four people falco ***** fox's kills because the horizontal kicks are easier to clear people with. Free for all is lame and all about stealing kills.
Marth is one of the characters that when all your friends are newbs pretty much whoever plays Marth will win. All you need is that big ol c-stick. But then as people start to use characters like fox and sheik sometimes they can just get too fast for you and out-maneuver. Of course then once you adapt to that style of play Marth becomes one of the most fiendish jerks out there. Edge-camping with Marth is a dream. If you can hit someone at like 60 damage with the tip of Marth's sword... well... they're going to have a rough time. I speak particularly as a fox player. A good Marth whoring the edge is pretty much one of my worst fears. I have nightmares. And once again in a four person free for all Marth's huge reach/throw lead to many stolen kills. You just make sure you're always the third man in with that big sword.
Sheik can be very fast and obviously has a lot of juggling potential. I play as fox though and don't know anyone who plays sheik around here good enough to beat me. I know that sheik has a lot of potential but in all truth most of the time I think fox has the upper hand. Sheik and fox are both very much technical players who require a tonne of skill to master but the rewards are massive. Sheik is very good at getting back from the edge against campers.
Falco is fast and technical like fox but his moves just don't go together as well as foxes for fighting most people. If only his shield deflected down like fox's then falco would be so frightening. He does have big chickenlegs though and if timed right they hurt. His splits are fast and after a shorthop l-cancelled down aerial they can pretty much clear fox anywhere above 60 damage near an edge. The trouble is that although he excels against fox to a certain degree he struggles with others and his shield and smaller firefox are a liability. He is still quite powerful though and much better for picking people off in free for all with his big sidekicks.
Surprisingly Jigglypuff can be quite a good character. I can't imagine it working well in a tournament setting but fast-fallers can really get pissed when all you need is one grab and they're dead even at 0 damage. Great combos that are super annoying and a lot of great smash and a-button attacks. Let's not forget the ultra-risky but oh-so-stylish twirling jiggs ball of doom move. That thing really smarts.
Pikachu actually has a lot of good aerial moves, smash moves, and a good up + B move. Pika is definitely not what he used to be in the 64 version (a cheap edge *****) but he is legitimately a decent fighter.
Cpt. Falcon isn't amazing but he's got decent moves and power and in free for all it's great to pick people off with a surprise falcon punch. Not going to do very well 1 vs 1 with a fox or marth though.
Believe it or not I've also seen Bowser played surprisingly well... same for DK. My one friend seems to stick with heavy characters and he is just really great with the timing and predicting others' play styles. Bowser has a lot of good moves if they are used properly. He's just a big slow lug so you've gotta be extra quick.
THE ICE CLIMBERS... They are among the most annoying to fight especially in a free for all. They interrupt your attacks constantly, have huge smashes, and if you have a friend who knows how to wobble/infinite it can make for a frustrating match up.
On he flip side of the coin... what were they thinking when they made pichu?