What you are looking for is a set of cards that has snap and does not bow easily when being shuffled. You don't want them to be too expensive so when people spill drinks on them or bend them by accident then you can replace them easily.
Lately I have been enamored with Cancelled Casino Cards. These cards come with either a hole punched in them, the corners cut off or a black mark on the side but the casinos swap cards out really fast so I've never gotten a mangled deck.
No matter what type of card you get, make sure you have at least 2 decks that have different back designs or colors so you don't get the decks mixed up. You might want to consider cards like the Bicycle Playing Card Lot which has 9 decks of playing cards in blue, red pastel, Silver Reverse, etc.
It's also important to count the cards, especially if they are a casino used deck, sometimes they get mixed up so there are more (or less) than 52 in the deck. This can make for some fun times when two players have the six of diamonds on a show down.
Various Card Stuff: