The hot tub is by far the best value in the game because it contributes to three motives (four, if there are other Sims in the tub). It can be used to replace any fun/bath objects and it's also a great way to put visiting Sims in a good mood to increase the likelihood of a positive result from an interaction. Even though it is expensive, it is recommended to buy one as soon as possible after starting a game. (Just make sure that you have a phone to call the repairman when it breaks.)