Which expansion pack can you absolutely NOT live without?

Seasons. It really adds to the world and wouldn't expect it to come with the base game (that would make it even more expensive), but maybe one of the first expansions instead.

I like to see how they react when outside. Slip in the mud, play in the snow or rain, etc.....now with Islanding Living coming out, they can tan and burn without mods, so it will feel more complete. Right now it is winter in the game so my sims had a snowball fight and built snowmen (though they been too busy caring for a toddler, having another baby on the way and working to really be out too much lately).