Carcassonne is an excellent 'gateway' game for introducing people to the boardgame phenomenon. Simple enough that you can explain the rules to an eight year old and start playing in five minutes. Also, it has just the right amount of luck and strategy to be fun for casual settings. I highly recommend it. You can keep making it more interesting by adding expansions later too.

Co-operative games are a tonne of fun but they tend to have complex mechanics and a bit of a set-up time in the beginning. This may be a discouraging factor but they are a lot of fun because everyone comes out of it with a sense of accomplishment after having done something together (if you win of course).