NL Ring is my game so my perspective is going to be slightly skewed towards that.

Here's the fortes of the different choices you have IMO.

LHE ring: Good if you are mathematically inclined. There is almost always a "right" play for a given situation. All you do is make plays consistant with a positive expectation and you will earn money. Theres very little psychological warfare and bluffing in limit. Also you can't lose all your money in one hand. In limit, your success depends on making as few small mistakes as possible, as there really aren't that many big mistakes.

NLHE ring: Like limit, you have to make plays that give you positive expectation, but its different in that there are usually a number of good plays for each situation. Aggression is a rewarded a lot more in NL, as the threat of losing a whole stack can win you many uncontested pots. This means that even if you don't catch cards you still can make money by reading weakness in your opponents and exploiting it. What characterizes NL is you can lose (or win) a lot of money in a very short time. NL is a lot more of a psychological warfare than limit.

SNG's: If you like an ever-changing, adapting, and competitive game, then SNG's may be for you. They require that you outwit your opponents in an escalating blinds situation, and usually you have to play a short stack. SNG's swing badly, however, so you need a lot of money to play them (like 50 buyins).

MTT's: Obviously they are like SNG's except they take longer and offer the reward of winning a lot more money. However the swings are even worse than sngs. You will often go for weeks without a big win, even if you are a good player (you will probably need at least 75 or 100 buyins to play mtts).