I actually have 2 favorites, but OP wanted THE MOST favorite, lol. I chose Eco Lifestyle for the simple reason I thought the ep was a load of cr*p when it was announced, but after playing it, most of my sims will have something to with the gameplay. Most sims either have a juicer, candlemaking table, a fabricator and the recycler. I have made all my community lots recycle places. Needless to say the pack has a great replayability for me...I even like the NAPS.
Second choice would have to be Seasons...who in the world doesn't have weather. :D