EP: Get Together is the most essential for me. I use the club system all the time, I love the dramatically expanded nightlife game play. Most importantly, I can't survive without Windenberg. Each neighborhood has its own character, making it feel like you have four (smallish) worlds for the price of one.
GP: This is hard, because I'm factoring the loss of so many EPs in my calculation. I would normally say I use Parenthood the most, but if I'm losing Sulani and San Myshuno, the lack of a world is a deal breaker. I'm going to have to say Strangerville for the world, and the ability to spy on people. Very hard to choose between that and Werewolves, though.
SP: Nifty Knitting. Didn't even have to think about this one. Now that I have Etsy in my game, I can't do without it.
Kit: I like Courtyard Oasis the most normally. The problem is that I use it as a booster pack for the City Living BB. Seriously, challenge yourself to make a room that only uses City Living and Courtyard Oasis assets. You'll see how well they blend together. If I'm losing City Living, I'll have to go with the only other kit I own, Desert Luxe. At least it ties into the contemporary themes available in base game and Get Together.
Whew! That was hard.
