Firstly, very good question. These 3 cards were banned from Modified during different periods of time
Sneasel
Neo Genesis Rare.
(Theres already a pic, so I won't bother with another one).
Most importantly, is this cards brutal attack. Often the player would aim to get this card out as fast as possible and have the energy needed for Beat up, by the 2nd turn (Using whatever search/draw cards necessary) whilst filling there bench.
With a full bench (very common) the card could achieve 120 on its second turn, not including Dark Energy bonus and pluspowers. Realistically the card would achieve 60 - 80 damage with each attack, but considering the energy cost it was still effective and able to KO most early starters.
On top of this it has a free retreat, no weakness and often 3-4 are played in a deck to manipulate them.
Because of the classic '1 in 3' holos in a pack ratio, there were numerous Sneasels in the game and were easy to get hold of. Eventually it was a scenario of play Sneasel or be beaten by Sneasel.
A few months after NGs release WotC or whoever it was at the time had to step in and prevented it from being played in modified tournaments for fairer games.
Slowking
Neo Genesis Holo
Of course trainers are a critical part of a players deck, so when Slowking came along, its Mind Game Power not only negated half of there opponents trainer cards but it also returned them to the top of their deck so they could hardly draw anything new.
As well as that, Mind Games power was cumulative (more than one could be used at a time), making the trainer card % often 25% or 12.5%. The game for the opponent would often come to a stop and abrupt loss.
The attacks of this card aren't any good, so Slowking would often sit on the bench, busily annoying your opponent and avoiding half the GoWs that came its way.
This was noticed after it completely dominated the Worlds Championship 2002 and the cards reign came to an end.
(Interestingly, the Slowkings power is actually a mistranslation from the Japanese- it should be active to be played. But WotC banned it rather than correct it).
Point of interest though, both cards made the 'Sneaking' deck, before there banning, which is currently now one of the best unlimited decks.
Also I made a mistake over the ER thing. When Expedition was released they introduced Energy Removal 2, which requires a coin flip to discard an opponents energy card of your choice. Some players believed / thought that they could get away with playing the original Energy Removal (which does not require a coin flip) on the ground that it was a reprint.
PUI/WotC made it clear that ER was not allowed in modifide, so it wasn't banned since it wasn't legal to begin with.
