This.
As an FYI, I have been a creeping "member" of this forum for many years now but finally decided to register, and it seems like the same debate comes up every time we go through this exercise of ranking our top prospects (which of late has been annual).
I ask myself what the point is of ranking our prospects on this board, and I always come up with something that relates to how an NHL GM would think, since most of us wish we could be a GM, or we at least enjoy thinking about the same things that an NHL GM would think about. Ranking them helps us assess any potential trades that involve our prospects, and helps us reconcile our feelings when one such trade sends a prospect elsewhere.
In order to look at the hierarchy of our prospects the way that an NHL GM would look at them we ought to take a comprehensive perspective instead of a one-dimensional one. I have always thought that this multi-dimensional perspective is captured by simply thinking about a prospect's true trade value, provided that there is full disclosure on the prospect and while ignoring potential synergies. This is something that we all likely do subconsciously when thinking about trade value, where we attribute different values to different factors in the equation. Let me expound on some of these factors.
Only looking at potential ceiling can be misleading, because the more time that has passed without seeing substantial progress in the prospect's developmental trajectory, the greater the risk of the prospect not reaching their potential becomes, and thus their trade value begins to decrease. In a similar manner, the closer a prospect is to their ceiling, or further along their developmental trajectory, the less vague their capabilities become, the less risk involved, and their trade value will increase proportionately.
Only looking at current NHL readiness is also misleading, for obvious reasons, as top prospects that are "green," such as Tkachuk, may not be adapted to the NHL game currently, but will have a much higher trade value than a grinding 4th liner that is finally looking to be pushing for an NHL spot. That being said, there is value attributed to the reduced risk of a prospect showing greater NHL readiness. At some point, potential ceiling and NHL readiness cross over the same trade value, where a lower risk and lower potential reward is equivalent to a higher risk with a higher potential reward.
Mathematically these two factors can be expressed something like the following:
Potential Ceiling (1-10) -
Current Risk of Not Reaching Ceiling (1-10) = Trade Value
Of course other factors are considered in a trade, such as how much of an impact could the prospect have on the club due to their position (the whole Goalie>Defenseman>Centre>Forward relationship), or even longevity of the prospect (how long will the prospect provide a certain benefit to the team, i.e. the prospect could have a high ceiling but have a history of injuries, and thus increasing the risk of the prospect).
Maybe these factors are more of a coefficient in the equation, such as:
(
Potential Ceiling (1-10))*
Positional Impact Factor -
Current Risk of Not Reaching Ceiling (1-10))*
Longevity Factor = Trade Value
Another, less important factor for a GM is how much of an impact the prospect could have because of the current structural make-up of the team (whether giant holes are being filled at a certain position or whether great depth is simply being added to in said position). I think factors like this are probably secondary for a GM when considering trading away a prospect since most
competent GMs will seek to maximize the value of the return for their assets (see also; Drafting Best Player Available), instead of getting fleeced because they put themselves in such a desperate situation where they need a certain role or position filled (
*cough* Edmonton *cough* 
). I would consider this factor a synergistic factor, much like the potential synergies with line-mates, which should all be ignored.
There are a lot of contributing factors that could be considered when trying to determine our top prospects; but in short, it comes down to potential risk versus potential reward, as that is how GMs assess trade value. This way we can come up with a valid, relevant, and useful prospect hierarchy. (Sorry for the essay...)