Slowking is a rather neat support Pokemon that gives you a benefit when your Pokemon are KOed. Statwise, Slowking has the old standby of 80 HP for a Stage 1, +20 Weakness to Psychic (anyone else still finding it odd that Psychic is weak to Psychic?), no Resistance, and 2 for a Retreat Cost. His Poke-Power, Trump Card, lets you get any card from your deck when a Pokemon of yours is KOed during your opponent's last turn (note that you have to activate Trump Card during your turn, and not immediately when the Pokemon is KOed). Trump Card provides great advantage in allowing you to perhaps get that one card you need to turn the game around. His sole attack, Psych Up, deals a basic 30 for a Psychic and Colorless Energy, but goes up to 60 damage if you use it again, making it an ok attack if you really need to use it. Overall though, it's best just to leave Slowking on the bench and to let him do his thing.
Modified - 7/10 (I think this has advantages as a good tech card, but being a Stage 1 hampers it a bit and it does fill up what might be crucial bench space later on)
Limited - 9/10 (His attack is better here, and Trump Card is increadibly useful in getting that 1 Supporter or evolution cardyou drafted and won't usually draw into)
Unlimited - 1/10 (No real use here, he's just dead bench weight and decks usually move along quickly enough)