Generally, almost everything that have something to do with Satellite admins count in some way.
Pegasus Magic in all its incarnations. Time it so your opponent can't block/evade it and you have an undodgable freezing attack. The same also goes to the Giga version. Both deal huge damage; however, the Giga doesn't do as much damage as its counterparts, but the freezing makes up for it.
Leo Kingdom GX, which deals a tremendous amount of Fire damage and breaks guards.
Dragon Sky GX is the worst offender. It hits 10 times, deals massive damage even when unboosted, blows away Auras, and to make matters worse, it's wood elemental, meaning that it doesn't give the enemy Mercy Invincibility.
The Blank Cards. You'll find 10 of these over the course of Star Force 2, and if you know the right codes, you can instantly turn them into practically any card in the game. Even Giga Cards. Even if they're from the first game. Capcom was originally going to sell the codes, but some people got a hold of them and leaked them over the Internet, defeating the whole purpose.
They were so bad that some releases remove them entirely and replace them with about 2000 zennys. Which leads to some questions when Geo is basically stealing $20 notes from his bosom friends' bedrooms.
The Favorites system in the first 2 games. You can select six (four in the second game) cards to be your "favorite" cards. What's the advantage of doing this, you ask? You can use it with any card on your screen. But wait, there's more! Anything can be a favorite, even if it's a Giga. The White Cards that replaced them in the third game are nowhere near as bad.
Actually, if you have Wi-Fi and some Brothers, the White Cards can be exploited to be just as bad. You can turn on their cards to either Multi-Noise or use the card roulette. Doing the latter will randomly select one of their White Cards...or if you're lucky, you get their Giga card instead.
Gemini Noise, in the third game. It adds a paralysis effect to all sword cards. In multiplayer, it can be used to easily lock onto any enemy and stun them for a long period of time, while dishing out lots of damage. Combine with the Bushido series, Spinblade series, and basically any other sword card aside from a couple for best effect. It gets worse when you combine it with Wolf Noise, which buffs your sword card power.
Ophiuca Noise in the same game. Normal cards confuse, your charge shot leaves grass problems, and +20 on all wood chips. Not bad so far, right? The problem here is the fact that wood chips don't flinch, which means they're spammable. Most of them hit multiple times. It also buffs a near Game Breaking Mega Chip and a very Game Breaking Giga Chip (Dragon Sky GX) mentioned above. It also buffs sword chip by making every hit leave targets confused.
Players of Black Ace get to use Black End Galaxy when they Finalize. It deals 500 damage, can't be avoided, and leaves any survivors immobile.
Meanwhile, Red Joker players get Red Gaia Eraser, which does 600 damage if the initial blast hits. Sadly, it doesn't have any status effects.
Tribe King in the second game. Oh god, Tribe King... It doubles the strength of every card, buffs up Wood, Electric and Fire cards even further, gives Super Armor and Side Select, and also the card charging ability of your game. For Ninja players, this translates to Squall 3, a multi-hit Wood attack, dealing 640 damage, piercing invisibility, and locking on anywhere on the field.
Bushido in the third game. It deals 50 more damage for the more Bushido cards you're using in a given turn, and also changes the range of all of them. If you have 3, then they get LifeSword range and the ability to ignore invisibility. The X version of this card deals 200 damage by default, so that's 900 damage with insane range right there. And then if you've got Gemini Noise, that's 900 damage that paralyzes!
Rogue Noise boosts all sword damage by 50, making Bushido and the SwordFighter series of chips extremely powerful.
The Best Combo system in the first game was by far, the most broken mechanic in the entire series, essentially allowing you to save a sequence of consecutive standard card attacks, and turn into a card that would cause you to repeat said attacks in an unavoidable, time stopping combo. With the use of grass or ice panels to boost elemental damage, it was easily possible to create a combo dealing well over 1K damage, and better (or worse) yet, you could receive one of these cards from each one of your brothers.