Would you start developing a service based on a great idea then finding a market for it or by market research then finding the killer idea?

Good question, sir. I feel if the idea is great, it will create markets by itself. So no, the product or service that you call a "killer app" would not only grow the market and minimizes the market research.

No comes the second way of doing it, researching the market first and then developing the idea. Here there are 2 points:

1.Others (competitors) have too researched the market at varying degrees, so the USP of the idea would be copied soon.

2.I somehow the idea is not executed well, all the efforts of market research are wasted.

So definitely, I would bank on the idea more than market research. And anyways, killer ideas only come up when we know the market fairly well.