Best moves are when you can add assets at a reasonable cost. A basic cost/benefit analysis to assess the ratio of potential benefit to actual cost.
As such, adding Matt Cassel provides the potential for the most benefit for a low cost.
Adding free agents for a modest cost would rank second. Felton and Harvin fall into this category. I'm dubious of Incognito to be sure.
Keeping your own valuable free agents would come next, i.e. Jerry Hughes.
The worst moves are the reverse when you risk too much for the expected benefit. It is hard to say which moves fit into this category. Losing Spiller for nothing might have been a mistake long term. Trading away a young potential All-Pro linebacker might be a mistake long term too. Hiring a loser head coach from the Jets (4-12 last year) was another big risk. We don't know yet how these moves will pan out.
The trade for McCoy and the signing of TE Clay both were high risk because the cost was so high for them. They may turn out to be excellent moves, but that remains to be seen. The Bills are obviously frustrated by the last 15 years of futility and failure, so it is refreshing to a lot of fans to see them making bold risky moves to turn the franchise around. Time will tell.
What is for sure is that the Bills probably have sold more tickets for per victory than any team in the league this century.