As far as items go, here's what I think, in order of imbalance caused (with notes on whether it's caused by the item itself or from a character-to-character balance basis IE one char gets more use out of it than another) from from most imbalanced to most acceptable and reasons why.
1) Item -
THAT FUCKING FAN - Many characters can combo infinitely with no hope of recourse.
NO. There is NO reason to have the fan on. Ever.
EVER.
2) Character/Item - Smash Balls - Transformation Final Smashes last entirely too long. If you catch someone at about 50% when you grab the smash ball, you can usually ring them out once, and have enough time to ring them out again even if they wait out the entire floating platform. There's a significant imbalance between a character's ease of getting a smash ball and their power as well (hi ness/lucas.)
3)Item - Super Star - I think we need no comment here. A character that can play a purely aggressive game with no fear of retaliation is a no-go.
4)Item - Clock - ZA WARUDO. Toki wo tomare. Then you walk toward them, grab them, throw them down, jump up and ring them out with an aerial. (Samus, for example, down throw, hop up, back air, jump off the stage, down air) Because while time slows down, overall momentum remains unchained. A ringout is a ringout is a ringout is a ringout. The chance of backfiring isn't really big enough to make you think twice about grabbing it and winning.
5)Item - Metal Box, Mushrooms, Lightning Bolt - Changes in size change the size of projectiles and explosives, change your knockout capability and your ability to knock people out. They turn the tide far too much. The Metal Box isn't quite as bad but it still pretty much makes you immune to ringouts until it expires.
6)Item - Hearts - The amount of time it takes to rack up 100 damage is enough to say this is pretty unfair.
7)Item - Assist Trophies/Pokeballs - Just grab 'em and use 'em. They most often home in and their ring out potential and lastability for some of them is just insane. If you could pick which assist trophies to leave on I'd say some of them were okay, but as it is.. Not really. (You get the NINJA TWINS/LATIOS and you might be set for a win, OR !!!!!! you get the mayor/goldeen.)
8)Item - M Tomato - While not as bad as the Heart, this is still pretty bad. Not to mention, if you get lucky enough to get two of them in a row, that's as good as getting one heart!
9)Character - Hammers - Stop! Hammert-.. Erh. They're kinda like the super star but instead of invincibility you just have an even better ringout! Golden Hammers should be off, period, but hammers themselves are bad enough. They give a significant advantage to fast characters since A: they can move around with it better and B: they can run away from other people who have it better. Additional advantages given to ranged characters since they're the only ones who can really safely counter it and even ring out their opponent even after picking up the hammer. EX1: Fox vs Ike - Ike will not be able to catch Fox if he gets the hammer. Additionally, Ike will not be able to run away if Fox gets the hammer. EX2- Fighting Samus, grabbing the hammer is a
DEATH WISH. Hope you weren't planning on avoiding that short hopped charge shot.
10)Item - Cracker Launcher - Probably the only item here I am sort of on the fence about. The item itself is incredibly powerful if your aim with it is fantastic, but it's only mediocre if it isn't. It's safer to just leave it off. (If you're good enough, you can easily juggle someone straight out of the top of the stagebox, often with enough shots left to do it again.)
11)Character - Exploding Crate - Fucks over anyone that might explode it by accident when it plops in front of one of their attacks that explodes it. The item itself isn't very bad but it can be frustrating to have someone blast it as you're trying to recover and it's near the edge of the stage, blowing it up and sending you flying.
This list is getting pretty long, but I think that's pretty much it for the obnoxious stuff. Moving onto items I'm mostly fine with when playing for fun, in no particular order.
Item - Deku Nut - It's kinda like a weaker, blockable stopwatch. Not nearly as bad.
Item - Pitfall - Like an even weaker Deku Nut, because it reduces knockback. Mostly acceptable.
Item - Home Run Bat - Definitely weaker than its Melee counterpart. No way you'll ever home run ANYONE with this bat. That aside, it's an incredible throwing item.
Item - Beam Sword - Beam sword is the new home run bat! Incredble range, decent to pitch. Some chars get more use out of it, but not to the exceptional degree of the Hammer.
Item - Fire Flower - The fan, but weaker and FAR easier to get out of. It also runs out. This one is okay by me.
Item - Warp Star - Predictable, dodgable, and leaves you vulnerable. The only reason to turn this off is if you ABSOLUTELY HATE EVERY ITEM EVER MUST HAVE THEM OFF.
EXPLOSIVES
Item - Smart Bomb - The coolest, biggest, most over the top explosion to ever be seen in Melee. This one's a keeper just because of how awesome it is! It also doesn't ring out many heavies until much higher percentages, you can DI your way back to the stage pretty well. Also hilarious when someone screws up and gets caught in it after they throw it, only to die. Definitely does not cause a significant imbalance in use between characters.
Item - Motion Sensor Bomb - Annoying. That's about it. It's fine.
Item - Bobomb - Lost a lot of the explosive force it had in Melee, it's not nearly as bad now.
Item that MUST REMAIN IN AT ALL TIMES:
Sandbag - Seriously.
