I think as far as variety of differing effects goes, the DP (and by extension Platinum) era was king in that respect. (I never played during EX era and they probably had a lot of interesting effects too, but I'm going to speak from personal experience.)
With the DP cards, I felt like most of the cards went out of their way to have new and interesting effects, just for the heck of it! Here are just some of the many examples:
~Torterra LV.X, Infernape LV.X, and Empoleon LV.X had amazing Poke-Powers (GoW when behind, draw 3 discard 2, or knock off 2 cards in opponent's hand). Infernape's Flare Up attack was also incredibly interesting - RR for 150, but you had to return exactly 8 R energy from your discard to the deck.
~Dusknoir DP - Gets rid of 1 opponent's Benched Pokemon completely, provided the opponent's bench is at 4 or more
~Electivire LV.X's Pulse Barrier - Wiped the opponent clear of stadiums and tools, and if he did, he got 1-turn immunity (repeated in Flygon RR)
~Honchkrow MT - Bench-sitter that let any Murkrow you had use Honchkrow's attacks at no cost (Honchkrow LV.X also had a cool attack that reclaimed 1 card from your discard upon knockout)
~Mr. Mime MT - Blocked damage from opponents that had 2 or less Energy cards on them (repeated in Dusknoir SF)
~Alakazam MT - Can negate an opponent's Poke-Power upon use, by discarding 2 cards from your hand (repeated in TGI Power Spray)
~Gallade SW - Flipped over prize cards to increase damage
~Gardevoir LV.X - Knocked out the Pokemon with the least HP left
~Claydol GE - Never-ending draw power (repeated in Magnezone Prime)
~Darkrai GE - Sleep Hole did more damage to a sleeping target, and also put you to sleep if you weren't already (repeated in a few cards)
~Darkrai LV.X - +10 damage for all your {D} energy awawawawawawa
~Phione MD - Can evolve any Pokemon in play straight from your deck (repeated in Spiritomb AR)
~Porygon-Z LV.X - Mode Crush got rid of all your opponent's Special Energy (repeated in so very many cards since)
~Garchomp LV.X - Restore yanked ANY Pokemon out of your discard pile for free
~Starmie LA - A 50-damage snipe to anything with a power or body (repeated in Gengar SF)
~Magnezone LV.X - The very first repeatable auto-paralyze attack (repeated in Flygon BCR)
~Machamp SF - Auto-KO'd a Basic Pokemon
~Dusknoir SF - Damage Even gained more power the more you hurt yourself - which Shadow Command facilitated
~Dusknoir LV.X - Ectoplasm would trigger upon death, becoming a stadium that hurt your opponents every turn, and returned to the hand (repeated in Flygon BCR)
~Regigigas LV.X - Destroys 1 of its own comrades to heal itself and keep on swinging
~Palkia G LV.X - Lost Cyclone cleans up the bench nicely
~Mismagius GL LV.X - Pretty sure it gained power with tools, my memory's vague here
~Flygon LV.X - 150 snipe to a LV.X? I'm okay with this
~Venusaur SV - Grants immunity to Special Conditions for Grass-types
~Gengar AR - Partial snipe AND a hit-and-run
~Spiritomb AR - Blocks items for both trainers while active
~Arceus Colorless - First attack can only be used if your field is full of Arceus, and it gives an energy to each one.
And that's just the Pokemon.
No wait, correction.
That's only some of the Pokemon. There's many many more I didn't mention!
HGSS made the game simpler, and BW dumbed it down even further. We are nowhere even close to the potential of interesting effects that Pokemon cards can have. We're actually going in the opposite direction.
This is why the DP-era is going to forever remain my favorite era of Pokemon.
