Let me give you some characteristics of the best niche market for you:
1. You already have a lot of knowledge of the market.
While you can succeed in a market by learning about it while marketing to it, it is very difficult and will slow you down greatly.
An "ideal" market will be one that you are already an expert in.
2. There is some good competition in the market.
If there is no competition then there is most likely no demand (99.99999% of the time). That means you will have a very hard time even getting the attention of those in that market.
3. The market is not too large.
If your market includes every North American, or every male, or every golf player, then it is probably too large. You need to make it small enough so that the really big companies will not even bother.
Later on once you have "conquered" that market you can expand into others, eventually growing into a multi-million dollar organization.
4. The market is not too small.
Make sure there are enough people in the market to sustain your efforts! If you are marketing to left-handed red-headed women golfers who have problems with using their sand wedge you are probably thinking too small!
5. You have a PASSION for the market.
You must have a passion for the market you are going into. When you show passion others that you are marketing to will feel that and will trust you far easier.