I usually play casual games of 4-5 players with people who are new to the game, or generally not gaming people; I own Base and Seaside as well as Walled Village. The cards people find most fun from this selection are generally Council Room (I've never met anyone who doesn't love this one), Island (it's a kind of deck thinning that people usually enjoy and understand the benefit of immediately), and Smugglers (an interaction that is always entertaining). Moat/Lighthouse, Laboratory, Fishing Village and Smithy are also popular. A set with any of those cards is likely to go down very well in a casual setting. Interestingly, I have found that certain powerful attacks (e.g. Ghost Ship, Ambassador) are hated, while others (e.g. Witch) are enjoyed. Cards like Treasure Map, which are generally not worth it unless you have a specific plan for them, are not good in casual games (this is probably why Ambassador is disliked).
Although I play the other Dominion sets with a gaming group, I don't think any of them are as good as Base and Seaside for entertainment value and friendliness to new players and non-gamers. Here's hoping that Dark Ages has a bunch of cards that are fun and not-too-serious ...