Actually it really depends on what you want to do...
Sleepy just gave you the moneymaking ideas. Sure you can make a good bit of money by taking only gathering skills. This is not always true when it comes to near end game, when you can use crafting to rake in a lot of gold.
Taking 2 gathering professions is just fine, and then once you hit about level 40, then just drop one and pick up a crafting tradeskill, level it up and then you will be making plenty of money.
Personally, I like engineering for all of its unique toys. As for leatherworking, you will definately benefit from it when you need some new armor. Sure, it can be a money sink, but not once you learn to craft some of the better things.
It is just up to you, do you want to make money quick, do you want a skill that can help you along on your journeys, such as Engineering for the jumper cables or such, or what? You can always try em all...