It is easier to launch a brand for a niche market with no competition or one that has enormous potential market share but lots of competition?

I believe it is easier to build a BRAND when there's an existing competition it makes your life easier to identify gaps & needs then you go with perfect positioning. Saying this drive me to the other corner it is really difficult to launch a brand in a niche market because here you are not only establishing the brand but you also creating the Platform where you should double your work to own the platform and to survive with your new born brand. 

It is equally difficult as the difficulty to launch a brand is not correlated to the quantity of competition.

Ultimately it depands on the starting capital.

If you want to launch a brand in a huge market were the top3 players are recording revenue of billions of USD we are forced to invest a lot of money to raise your brand awarness, your market share and so on. Plus we need more than one-two shops in order to make your brand count.

If you want to launch a brand in a niche market it's easier. I have work in2 new brands for a niche market and with investments under100.000 USD you can launch a brand and sustain it for it's first years and meanwhile create the market for your product, educate your clientele, launch trends and market campaings without any real huge competition.