Every once in a while I'm successful enough with a young Sim that the game lets me choose their new trait when they grow up.  (It doesn't happen as often as it could, I am ashamed to admit.)   The question always becomes, which trait should I choose?

If you have an idea for where you want your Sim to go in life, obviously there are traits that can help with that.  Artistic Sims paint better, Genius Sims learn the logic skill faster, and so forth.  But let's say you have already chosen the important trait, and now you just need something to round out the roster.

I have found that there is a short list of traits which are handy to assign, because they are so easy to please.  Every Sim gets wishes which are related to their traits.  Fulfilling these wishes gives your Sim more Lifetime Happiness points, which can be traded in for awesome stuff!  (If you haven't checked the list since you installed World Adventures, be sure to do so.  New rewards have been added, including "Never pay bills," which is pretty effing great!)

Here are some of the most and least successful traits I have encountered so far:

Best Traits

Handy: Handy Sims will immediately want to repair anything that breaks.  As we all know, stuff breaks a lot!  Not only can a Handy Sim repair it for free, but they get Happiness Points for doing so.

Hates The Outdoors: Not so great if you want your Sim to be a gardener or an angler.  But on the up side, whenever a Sim goes outside, they will get a wish to go back inside.  That's an easy 250 happiness points right there!  

Kleptomaniac: Let's face it, it's fun to steal stuff.  And whenever you steal something you can either keep it (awesome!) or mail it back and get happiness points.  Win-win!

Neat: This is one of the best traits to grant your Sims.  Neat Sims are constantly aching to clean stuff.  Which is handy, because stuff is always getting dirty!  Not only are you constantly racking up happiness points just by cleaning, but Neat Sims also get a special "clean the entire house" action.  Extremely handy to have around!

Worst Traits

Bookworm: This trait is handy because your Sim is easily entertained.  But from a happiness perspective it's hard.  Your Sim never just wants to "read a book," it's always "read 5 books" or "read 20 books."  It's nearly impossible to fulfill all the wishes of a Bookworm Sim!

Easily Impressed: This trait is totally useless.  That must be why the game always assigns it for my Sims.  As punishment.

Absent Minded, Brave, Inappropriate: There's an entire category of traits which affect your Sim's behavior, but don't offer any clear advantages other than a few extra actions on the menu.

 Can't Stand Art, Technophobe, Clumsy, Commitment Issues, Dislikes Children, etc: these "negative traits" are supremely annoying.  It's just one more thing for your Sim to complain about.  Try to avoid traits that are defined as hating something, unless you're going for the humor value.