The best opportunities are those where you create a new marketplace - an inherent result is that you own the market, initially at least. Big risk here though is overestimating the potential size of the market.
Second best is to attack an existing market in a novel way. You need to be able to communicate the benefits of your methodology to an existing customer base that has been conditioned to expect businesses in that market to operate in a certain way, and comparable with a specific set of metrics. Your novelty cannot ignore this scoring method.
The worst opportunities are the "me-too" models, where you attempt to compete against incumbents in an existing market by copying (what appear to be) their business models. Good luck with that.
As for me and my current idea (still quite gestational), I'm looking at a "second-best" opportunity. The market is known, huge, and essential. The novelty is (imho) pretty awesome. It'll need funding, but I believe the story is very compelling. Time will tell.
